Method for manufacturing semiconductor device

ABSTRACT

An object is to manufacture a semiconductor device including an oxide semiconductor at low cost with high productivity in such a manner that a photolithography process is simplified by reducing the number of light-exposure masks. In a method for manufacturing a semiconductor device including a channel-etched inverted-staggered thin film transistor, an oxide semiconductor film and a conductive film are etched using a mask layer formed with the use of a multi-tone mask which is a light-exposure mask through which light is transmitted so as to have a plurality of intensities. In etching steps, a first etching step is performed by dry etching in which an etching gas is used, and a second etching step is performed by wet etching in which an etchant is used.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including anoxide semiconductor, and a manufacturing method thereof.

2. Description of the Related Art

A thin film transistor formed over a flat plate such as a glasssubstrate is manufactured using amorphous silicon or polycrystallinesilicon, as typically seen in a liquid crystal display device. A thinfilm transistor manufactured using amorphous silicon has lowfield-effect mobility, but such a transistor can be formed over a glasssubstrate with a larger area. On the other hand, a thin film transistormanufactured using a crystalline silicon has high field-effect mobility,but a crystallization step such as laser annealing is necessary and sucha transistor is not always suitable for a larger glass substrate.

In contrast, attention has been drawn on a technique by which a thinfilm transistor is manufactured using an oxide semiconductor and appliedto an electronic device or an optical device. For example, PatentDocument 1 and Patent Document 2 each disclose a technique by which athin film transistor is manufactured using zinc oxide or anIn—Ga—Zn—O-based oxide semiconductor, which is formed into an oxidesemiconductor film, and which is used for a switching element or thelike of an image display device.

REFERENCES

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

SUMMARY OF THE INVENTION

A thin film transistor including an oxide semiconductor in a channelformation region has higher field-effect mobility than a thin filmtransistor including amorphous silicon. An oxide semiconductor film canbe formed at temperatures of 300° C. or less by a sputtering method orthe like, and a manufacturing process of a thin film transistorincluding an oxide semiconductor film is simpler than that of a thinfilm transistor including polycrystalline silicon.

There is an expectation for application of such an oxide semiconductorto liquid crystal displays, electroluminescent displays, electronicpaper, and the like by forming a thin film transistor including theoxide semiconductor over a glass substrate, a plastic substrate, or thelike.

As a method for manufacturing a thin film transistor, a method by whicha stacked structure is formed by a photolithography process using anumber of light-exposure masks (also referred to as photomasks) isemployed. However, a photolithography process includes a number of stepsand is one factor of largely affecting the manufacturing cost, yield,productivity, and the like. In particular, reducing the number oflight-exposure masks whose design and manufacturing costs are high is asignificant object.

In view of the above problems, it is an object to manufacture asemiconductor device at low cost with high productivity in such a mannerthat a photolithography process is simplified by reducing the number oflight-exposure masks.

In a method for manufacturing a semiconductor device including aninverted staggered thin film transistor, an etching step is performedwith the use of a mask layer formed using a multi-tone mask which is alight-exposure mask through which light is transmitted so as to have aplurality of intensities.

Since a mask layer formed using a multi-tone mask has a plurality ofthicknesses and can be further changed in shape by performing etching,the mask layer can be used in a plurality of etching steps to providedifferent patterns. Therefore, a mask layer corresponding at least twokinds of different patterns can be formed by one multi-tone mask. Thus,the number of light-exposure masks can be reduced and the number ofcorresponding photolithography processes can also be reduced, wherebysimplification of a manufacturing process can be realized.

A process for manufacturing an inverted staggered thin film transistorincludes an etching step (a first etching step) of processing asemiconductor film and a conductive film into island shapes and anetching step (a second etching step) of etching the conductive film andthe semiconductor film into a source electrode layer, a drain electrodelayer, and a semiconductor layer having a depression. The first etchingstep is performed by dry etching in which an etching gas is used, andthe second etching step is performed by wet etching in which an etchantis used.

As the etching gas, a gas including chlorine (a chlorine-based gas suchas Cl₂, BCl₃, or SiCl₄) is preferable. Alternatively, a gas obtained byadding oxygen or a rare gas (such as Ar) to the above gas may be used asthe etching gas.

As the etchant, a mixed solution of phosphoric acid, acetic acid, andnitric acid or an ammonia hydrogen peroxide mixture can be used.

An oxide semiconductor used in this specification is formed into a thinfilm represented by InMO₃(ZnO)_(m) (m>0), and a thin film transistor ismanufactured using this thin film as a semiconductor layer. Note that Mdenotes one or more of metal elements selected from gallium (Ga), iron(Fe), nickel (Ni), manganese (Mn), or cobalt (Co). For example, in somecases, M denotes Ga and any of the above metal elements other than Ga,such as Ga and Ni, or Ga and Fe. The above oxide semiconductor includes,in some cases, a transition metal element such as Fe or Ni or an oxideof the transition metal as an impurity element, in addition to the metalelement included as M. In this specification, this thin film is alsocalled an In—Ga—Zn—O-based non-single-crystal film.

Since an In—Ga—Zn—O-based non-single-crystal film is formed by asputtering method and then subjected to thermal treatment at 200° C. to500° C., typically 300° C. to 400° C. for 10 minutes to 100 minutes, anamorphous structure is observed as its crystal structure in an XRD(X-ray diffraction) analysis. Moreover, as for the electricalcharacteristics of the thin film transistor, an on/off ratio of 10⁹ ormore and a mobility of 10 or more at a gate voltage of ±20 V can beachieved.

According to one embodiment of the present invention disclosed in thisspecification, a gate electrode layer is formed over a substrate havingan insulating surface, a gate insulating layer, an oxide semiconductorfilm, and a conductive film are stacked over the gate electrode layer, afirst mask layer is formed over the gate insulating layer, the oxidesemiconductor film, and the conductive film, an oxide semiconductorlayer and a conductive layer are formed by etching the oxidesemiconductor film and the conductive film with the use of the firstmask layer in a first etching step, a second mask layer is formed byetching the first mask layer, and an oxide semiconductor layer having adepression, a source electrode layer, and a drain electrode layer areformed by etching the oxide semiconductor layer and the conductive layerwith the use of the second mask layer in a second etching step, whereinthe first mask layer is formed using a light-exposure mask through whichlight is transmitted so as to have a plurality of intensities, whereindry etching in which an etching gas is used is employed in the firstetching step, wherein wet etching in which an etchant is used isemployed in the second etching step, and wherein the oxide semiconductorlayer having a depression includes a region with a smaller thicknessthan a region overlapping with the source electrode layer or the drainelectrode layer.

According to another embodiment of the present invention disclosed inthis specification, a gate electrode layer is formed over a substratehaving an insulating surface, a gate insulating layer, a first oxidesemiconductor film, a second oxide semiconductor film, and a conductivefilm are stacked over the gate electrode layer, a first mask layer isformed over the gate insulating layer, the first oxide semiconductorfilm, the second oxide semiconductor film, and the conductive film, afirst oxide semiconductor layer, a second oxide semiconductor layer, anda conductive layer are formed by etching the first oxide semiconductorfilm, the second oxide semiconductor film, and the conductive film withthe use of the first mask layer in a first etching step, a second masklayer is formed by etching the first mask layer, and an oxidesemiconductor layer having a depression, a source region, a drainregion, a source electrode layer, and a drain electrode layer are formedby etching the first oxide semiconductor layer, the second oxidesemiconductor layer, and the conductive layer with the use of the secondmask layer in a second etching step, wherein the first mask layer isformed using a light-exposure mask through which light is transmitted soas to have a plurality of intensities, wherein dry etching in which anetching gas is used is employed in the first etching step, wherein wetetching in which an etchant is used is employed in the second etchingstep, and wherein the oxide semiconductor layer having a depressionincludes a region with a smaller thickness than a region overlappingwith the source region or the drain region.

The method for manufacturing a semiconductor device disclosed in thisspecification achieves at least one of the above objects.

Moreover, the second oxide semiconductor film used for the source regionand the drain region of the thin film transistor is preferably thinnerthan the first oxide semiconductor film used for a channel formationregion and preferably has higher conductivity (electrical conductivity)than the first oxide semiconductor film.

The second oxide semiconductor film has n-type conductivity and servesas the source region and the drain region.

Moreover, the first oxide semiconductor film has an amorphous structureand the second oxide semiconductor film includes a crystal grain(nanocrystal) in an amorphous structure in some cases. The crystal grain(nanocrystal) in the second oxide semiconductor film has a diameter of 1nm to 10 nm, typically approximately 2 nm to 4 nm.

As the second oxide semiconductor film used for the source region andthe drain region (n⁺ layer), an In—Ga—Zn—O-based non-single-crystal filmcan be used.

An insulating film may be formed so as to cover the thin film transistorand be in contact with the oxide semiconductor layer including thechannel formation region.

Moreover, since the thin film transistor is easily destroyed by staticelectricity or the like, a protective circuit for protecting a drivercircuit is preferably provided over the same substrate as a gate wiringor a source wiring. The protective circuit is preferably formed using anon-linear element including an oxide semiconductor.

Note that the ordinal numbers such as “first” and “second” are used forconvenience and do not define the order of steps and the stacking orderof layers. In addition, the ordinal numbers in this specification do notdenote particular names which specify the invention.

As a display device including a driver circuit, there are alight-emitting display device including a light-emitting element and adisplay device including an electrophoretic display element, which isalso referred to as electronic paper, in addition to a liquid crystaldisplay device.

A light-emitting display device including a light-emitting elementincludes a pixel portion having a plurality of thin film transistors.The pixel portion includes a region where a gate electrode of one thinfilm transistor is connected to a source or drain wiring of another thinfilm transistor. A driver circuit of the light-emitting display deviceincluding a light-emitting element includes a region where a gateelectrode of a thin film transistor is connected to a source or drainwiring of the thin film transistor.

Note that the semiconductor devices in this specification indicate allthe devices which can operate by using semiconductor characteristics,and an electro-optical device, a semiconductor circuit, and anelectronic appliance are all included in the category of thesemiconductor devices.

Further, by reducing the number of light-exposure masks, aphotolithography process is simplified, whereby a reliable semiconductordevice can be manufactured at low cost with high productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E show a method for manufacturing a semiconductor device.

FIGS. 2A1 and 2A2 show a semiconductor device.

FIGS. 3A to 3E show a method for manufacturing a semiconductor device.

FIGS. 4A1 and 4A2 show a semiconductor device.

FIGS. 5A to 5C show a method for manufacturing a semiconductor device.

FIGS. 6A to 6C show a method for manufacturing a semiconductor device.

FIG. 7 shows a method for manufacturing a semiconductor device.

FIG. 8 shows a method for manufacturing a semiconductor device.

FIG. 9 shows a method for manufacturing a semiconductor device.

FIG. 10 shows a semiconductor device.

FIGS. 11A1, 11A2, 11B1, and 11B2 show semiconductor devices.

FIG. 12 shows a semiconductor device.

FIG. 13 shows a semiconductor device.

FIGS. 14A and 14B each show a block diagram of a semiconductor device.

FIG. 15 shows a structure of a signal-line driver circuit.

FIG. 16 is a timing chart illustrating operation of the signal-linedriver circuit.

FIG. 17 is a timing chart illustrating operation of the signal-linedriver circuit.

FIG. 18 shows a structure of a shift register.

FIG. 19 shows a connection structure of a flip-flop of FIG. 18.

FIG. 20 shows an equivalent circuit of a pixel in a semiconductordevice.

FIGS. 21A to 21C each show a semiconductor device.

FIGS. 22A1, 22A2, and 22B show semiconductor devices.

FIG. 23 shows a semiconductor device.

FIGS. 24A and 24B show a semiconductor device.

FIGS. 25A and 25B each show an example of application of electronicpaper.

FIG. 26 is an external view showing an example of an electronic book.

FIGS. 27A and 27B are external views showing examples of a televisiondevice and a digital photo frame, respectively.

FIGS. 28A and 28B are external views showing examples of game machines.

FIGS. 29A and 29B are external views showing examples of cellularphones.

FIGS. 30A to 30D show multi-tone masks.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments are described in detail with reference to the drawings.However, it is easily understood by those skilled in the art that themodes and details herein disclosed can be modified in a variety of wayswithout departing from the scope and the spirit of the presentinvention. Therefore, the present invention should not be interpreted asbeing limited to the description of Embodiments given below. In thestructures of the present invention described below, the same portionsor portions having similar functions are denoted by the same referencenumerals in different drawings, and the description thereof will not berepeated.

Embodiment 1

A method for manufacturing a semiconductor device of this embodiment isdescribed with reference to FIGS. 1A to 1E and FIGS. 2A1 and 2A2.

FIG. 2A1 is a plan view of a thin film transistor 420 of a semiconductordevice of this embodiment, and FIG. 2A2 is a cross-sectional view takenalong C1-C2 of FIG. 2A1. The thin film transistor 420 is an invertedstaggered thin film transistor and includes a gate electrode layer 401,a gate insulating layer 402, a semiconductor layer 403, n⁺ layers 404 aand 404 b serving as a source region and a drain region, and source anddrain electrode layers 405 a and 405 b.

FIGS. 1A to 1E correspond to cross-sectional views showing steps ofmanufacturing the thin film transistor 420.

In FIG. 1A, an insulating film 407 serving as a base film is providedover a substrate 400 and the gate electrode layer 401 is provided overthe insulating film 407. The insulating film 407 has a function ofpreventing diffusion of an impurity element from the substrate 400, andcan be formed to have a single-layer or stacked-layer structure usingone or more of a silicon nitride film, a silicon oxide film, a siliconnitride oxide film, and a silicon oxynitride film. In this embodiment, asilicon oxide film (with a thickness of 100 nm) is used. The gateelectrode layer 401 can be formed to have a single-layer orstacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material which contains any of these materials asa main component.

For example, as a two-layer structure of the gate electrode layer 401,the following structures are preferable: a two-layer structure in whicha molybdenum layer is stacked over an aluminum layer, a two-layerstructure in which a molybdenum layer is stacked over a copper layer, atwo-layer structure in which a titanium nitride layer or a tantalumnitride layer is stacked over a copper layer, and a two-layer structurein which a titanium nitride layer and a molybdenum layer are stacked. Asa three-layer structure, it is preferable to stack a tungsten layer or atungsten nitride layer, an alloy of aluminum and silicon or an alloy ofaluminum and titanium, and a titanium nitride layer or a titanium layer.

The gate insulating layer 402, a first oxide semiconductor film 431, asecond oxide semiconductor film 432, and a conductive film 433 arestacked in that order over the gate electrode layer 401.

The gate insulating layer 402 can be formed to have a single-layer orstacked-layer structure by a plasma CVD method, a sputtering method, orthe like using a silicon oxide layer, a silicon nitride layer, a siliconoxynitride layer, or a silicon nitride oxide layer. Alternatively, thegate insulating layer 402 can be formed using a silicon oxide layer by aCVD method in which an organosilane gas is used. As the organosilanegas, a silicon-containing compound such as tetraethoxysilane (TEOS:chemical formula, Si(OC₂H₅)₄), tetramethylsilane (TMS: chemical formula,Si(CH₃)₄), tetramethylcyclotetrasiloxane (TMCTS),octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS),triethoxysilane (SiH(OC₂H₅)₃), or trisdimethylaminosilane(SiH(N(CH₃)₂)₃) can be used.

Note that before the first oxide semiconductor film 431 is formed by asputtering method, dust on a surface of the gate insulating layer 402 ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering is a methodby which voltage is applied to a substrate side using an RF power togenerate plasma on the substrate side in an argon atmosphere withoutapplying voltage to a target side, so that a surface is modified.Nitrogen, helium, or the like may be used instead of the argonatmosphere. Alternatively, oxygen, hydrogen, N₂O, or the like may beadded to the argon atmosphere. Further alternatively, Cl₂, CF₄, or thelike may be added to the argon atmosphere.

A region where the second oxide semiconductor film 432 and theconductive film 433 are in contact with each other is preferablymodified through plasma treatment. In this embodiment, the plasmatreatment is performed on the second oxide semiconductor film 432 (inthis embodiment, an In—Ga—Zn—O-based non-single-crystal film) in anargon atmosphere before the conductive film 433 is formed.

The plasma treatment may be performed using nitrogen, helium, or thelike instead of the argon atmosphere. Alternatively, oxygen, hydrogen,N₂O, or the like may be added to the argon atmosphere. Furtheralternatively, Cl₂, CF₄, or the like may be added to the argonatmosphere.

In this embodiment, an In—Ga—Zn—O-based non-single-crystal film is usedas each of the first oxide semiconductor film 431 and the second oxidesemiconductor film 432. The first oxide semiconductor film 431 and thesecond oxide semiconductor film 432 are formed under differentconditions, and the second oxide semiconductor film 432 has higherconductivity and lower resistance than the first oxide semiconductorfilm 431. For example, the second oxide semiconductor film 432 is formedusing an oxide semiconductor film obtained by a sputtering method inwhich the argon gas flow rate is set to 40 sccm. The second oxidesemiconductor film 432 has n-type conductivity and has an activationenergy (ΔE) of from 0.01 eV to 0.1 eV. Note that in this embodiment, thesecond oxide semiconductor film 432 is an In—Ga—Zn—O-basednon-single-crystal film and includes at least an amorphous component. Insome cases, the second oxide semiconductor film 432 has a crystal grain(nanocrystal) in an amorphous structure. The crystal grain (nanocrystal)in this second oxide semiconductor film 432 has a diameter of 1 nm to 10nm, typically approximately 2 nm to 4 nm.

By the provision of the second oxide semiconductor film 432 serving asan n⁺ layer, the conductive film 433 formed using a metal layer and thefirst oxide semiconductor film 431 serving as a channel formation regionhave a favorable junction, which allows more thermally-stable operationthan a Schottky junction. In addition, willing provision of the n⁺ layeris effective in supplying carriers to the channel (on the source side),stably absorbing carriers from the channel (on the drain side), orpreventing a resistance component from being formed at an interface withthe wiring. Further, by the decrease in resistance, high mobility can bemaintained even at high drain voltage.

The gate insulating layer 402, the first oxide semiconductor film 431,the second oxide semiconductor film 432, and the conductive film 433 canbe formed successively without exposure to air. By the successiveformation without exposure to air, the films can be stacked without theinterface therebetween contaminated by an atmospheric component or acontaminant impurity element floating in air; therefore, variation incharacteristics of a thin film transistor can be decreased.

A mask 434 is formed over the gate insulating layer 402, the first oxidesemiconductor film 431, the second oxide semiconductor film 432, and theconductive film 433.

In this embodiment, an example is shown in which the mask 434 is formedin such a manner that light-exposure is performed using a high-tonemask. A resist is formed in order to form the mask 434. As the resist, apositive type resist or a negative type resist can be used. Here, apositive resist is used.

Next, the resist is irradiated with light with the use of a multi-tonemask 59 as a light-exposure mask, so that the resist is exposed tolight.

Here, light exposure with the multi-tone mask 59 is described withreference to FIGS. 30A to 30D.

A multi-tone mask can achieve three levels of light exposure, so that anexposed portion, a semi-exposed portion, and an unexposed portion can beformed. In other words, a multi-tone mask is a mask through which lightis transmitted so as to have a plurality of intensities. One-time lightexposure and development process allows a resist mask having regionswith plural thicknesses (typically, two kinds of thicknesses) to beformed. Thus, the number of light-exposure masks can be reduced by usinga multi-tone mask.

Typical examples of a multi-tone mask include a gray-tone mask 59 a asshown in FIG. 30A and a half-tone mask 59 b as shown in FIG. 30C.

As shown in FIG. 30A, the gray-tone mask 59 a includes alight-transmitting substrate 63, and a light-blocking portion 64 and adiffraction grating 65 which are formed on the light-transmittingsubstrate 63. The light transmittance of the light-blocking portion 64is 0%. The diffraction grating 65 has light-transmitting portions in aslit form, a dot form, a mesh form, or the like with intervals which areless than or equal to the resolution limit of light used for the lightexposure, whereby the light transmittance can be controlled. Thediffraction grating 65 can be either in a slit form, a dot form, or amesh form with regular intervals; or in a slit form, a dot form, or amesh form with irregular intervals.

As the light-transmitting substrate 63, a light-transmitting substratesuch as a quartz substrate can be used. The light-blocking portion 64and the diffraction grating 65 can be each formed of a light-blockingmaterial which absorbs light, such as chromium or chromium oxide.

When the gray-tone mask 59 a is irradiated with light for exposure,light transmittance 66 of the light-blocking portion 64 is 0% and thelight transmittance 66 of a region where the light-blocking portion 64and the diffraction grating 65 are not provided is 100%, as shown inFIG. 30B. The light transmittance 66 of the diffraction grating 65 canbe controlled in the range of 10% to 70%. The light transmittance of thediffraction grating 65 can be controlled by controlling the interval andpitch of slits, dots, or meshes of the diffraction grating.

As shown in FIG. 30C, the half-tone mask 59 b includes thelight-transmitting substrate 63, and a semi-transmissive portion 68 anda light-blocking portion 67 which are formed on the light-transmittingsubstrate 63. The semi-transmissive portion 68 can be formed usingMoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The light-blockingportion 67 can be formed using a light-blocking material which absorbslight, such as chromium or chromium oxide.

In the case where the half-tone mask 59 b is irradiated with light forexposure, as shown in FIG. 30D, light transmittance 69 of thelight-blocking portion 67 is 0% and that of a region where thelight-blocking portion 67 and the semi-transmissive portion 68 are notprovided is 100%. Further, the light transmittance 69 of thesemi-transmissive portion 68 can be controlled in the range of 10% to70%. The light transmittance of the semi-transmissive portion 68 can becontrolled by choosing the material of the semi-transmissive portion 68.

The light exposure is performed using the multi-tone mask, and thendevelopment is performed; accordingly, the mask 434 having regions withdifferent thicknesses can be formed as shown in FIG. 1B.

Next, a first etching step is performed using the mask 434; accordingly,the first oxide semiconductor film 431, the second oxide semiconductorfilm 432, and the conductive film 433 are etched into island shapes. Asa result, a first oxide semiconductor layer 435, a second oxidesemiconductor layer 436, and a conductive layer 437 can be formed (seeFIG. 1B).

In this embodiment, the first etching step is performed by dry etchingin which an etching gas is used.

As the etching gas, a gas including chlorine (chlorine-based gas such aschlorine (Cl₂), boron chloride (BCl₃), silicon chloride (SiCl₄), orcarbon tetrachloride (CCl₄)) is preferably used. With the use of the gasincluding chlorine in etching, in-plane variation in etching can bereduced as compared to the case of using a gas without chlorine.

Alternatively, a gas including fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur fluoride (SF₆), nitrogen fluoride(NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂);any of these gases to which a rare gas such as helium (He) or argon (Ar)is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. The etching condition (the amount of electric power applied to acoil-shaped electrode, the amount of electric power applied to anelectrode on a substrate side, the temperature of the electrode on thesubstrate side, or the like) is adjusted as appropriate so that thefilms can be etched into desired shapes.

In this embodiment, an ICP etching method is employed and the etchingcondition is as follows: Cl₂ and O₂ are used; the amount of electricpower applied to the coil-shaped electrode is 1500 W; the amount ofelectric power applied to the electrode on the substrate side is 200 W;a pressure is 1.5 Pa; and a substrate temperature is −10° C.

Alternatively, the ICP etching method may be performed under thefollowing etching condition: Cl₂ (with a flow rate of 100 sccm) is usedas an etching gas; the amount of electric power applied to thecoil-shaped electrode is 2000 W; the amount of electric power applied tothe electrode on the substrate side is 600 W; a pressure is 1.5 Pa; anda substrate temperature is −10° C.

When the first oxide semiconductor film 431 and the second oxidesemiconductor film 432, which are formed using the In—Ga—Zn—O-basednon-single-crystal film, are etched under the above condition, the endportion of the semiconductor layer 403 can have a small tapered angle of5 degrees or less. In this case, the coverage of the film which isstacked over the semiconductor layer 403 can be improved. In addition,in the etching process, the end of the etching (also referred to as anend point) is preferably determined by monitoring the wavelengthcorresponding to each atom in the oxide semiconductor films while plasmaemission intensity is measured. This method makes it possible to controlthe etching so that the decrease in thickness of the gate insulatinglayer under the semiconductor layer can be suppressed and the etchingresidue of the oxide semiconductor films can be reduced.

When the etching is performed using a chlorine-based gas (Cl₂) to whichan oxygen gas (O₂) is added (preferably, the content of oxygen in theetching gas is set to be 15 vol % or more), in the case of using asilicon oxynitride film as the gate insulating layer 402, theselectivity ratio of the In—Ga—Zn—O-based non-single-crystal film usedfor the first oxide semiconductor film 431 and the second oxidesemiconductor film 432 with respect to the gate insulating layer 402 canbe increased. Therefore, the first oxide semiconductor film 431 and thesecond oxide semiconductor film 432 can be etched more than the gateinsulating layer 402, and the damage on the gate insulating layer 402can be sufficiently decreased.

Through the first etching step in which the first oxide semiconductorfilm 431, the second oxide semiconductor film 432, and the conductivefilm 433 are dry-etched, the first oxide semiconductor film 431, thesecond oxide semiconductor film 432, and the conductive film 433 areetched anisotropically. In this manner, the end portion of the mask 434is aligned with end portions of the first oxide semiconductor layer 435,the second oxide semiconductor layer 436, and the conductive layer 437,and these end portions become continuous.

In addition, since the etching rates of the end portions of the firstoxide semiconductor layer 435, the second oxide semiconductor layer 436,and the conductive layer 437 are different depending on the etchingconditions or oxide semiconductor materials and conductive materials,the tapered angles are different and the end portions are not continuousin some cases.

Next, the mask 434 is subjected to ashing. As a result, the mask isreduced in size and thickness. Through the ashing, a region of theresist mask, which has small thickness (a region overlapping with partof the gate electrode layer 401), is removed, so that divided masks 438can be formed (see FIG. 1C).

A second etching step is performed using the masks 438; accordingly, thefirst oxide semiconductor layer 435, the second oxide semiconductorlayer 436, and the conductive layer 437 are etched into a semiconductorlayer 403, n⁺ layers 404 a and 404 b, and source and drain electrodelayers 405 a and 405 b (see FIG. 1D). Note that the semiconductor layer403 is partly etched to become a semiconductor layer having a groove (adepression) and also having an end portion which is partly etched andexposed.

In this embodiment, the second etching step is performed by wet etchingin which an etchant is used.

As the etchant, a mixed solution of phosphoric acid, acetic acid, andnitric acid, an ammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2), or the like can be used. Alternatively,ITO07N (manufactured by Kanto Chemical Co., Inc) may be used.

The etching condition (etchant, etching time, temperature, or the like)is adjusted as appropriate, depending on a material used for theconductive layer 437, so that the films can be etched into desiredshapes.

For example, in the case where an aluminum film or an aluminum alloyfilm is used for the conductive layer 437, wet etching using a mixedsolution of phosphoric acid, acetic acid, and nitric acid can beperformed. Further, in the case where a titanium film is used for theconductive layer 437, wet etching using an ammonia hydrogen peroxidemixture (hydrogen peroxide:ammonia:water=5:2:2) as the etchant can beperformed.

For example, in the case where an aluminum film or an aluminum alloyfilm is used for the conductive layer 437, the first oxide semiconductorlayer 435, the second oxide semiconductor layer 436, and the conductivelayer 437 may be etched using a mixed solution of phosphoric acid,acetic acid, and nitric acid as the etchant of the second etching step.

In the second etching step, the conductive layer and the oxidesemiconductor layers may be etched using different etchants.

For example, in the case where a titanium film is used for theconductive layer 437, the conductive layer 437 is etched using anammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2) as the etchant of the second etching step,and the first oxide semiconductor layer 435 and the second oxidesemiconductor layer 436 may be etched using a mixed solution ofphosphoric acid, acetic acid, and nitric acid.

In the second etching step, the first oxide semiconductor layer 435, thesecond oxide semiconductor layer 436, and the conductive layer 437 maybe etched using an ammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2) so that a depression of the semiconductorlayer 403, the n⁺ layers 404 a and 404 b, and the source and drainelectrode layers 405 a and 405 b can be formed.

Through the second etching step in which the first oxide semiconductorlayer 435, the second oxide semiconductor layer 436, and the conductivelayer 437 are wet-etched, the first oxide semiconductor layer 435, thesecond oxide semiconductor layer 436, and the conductive layer 437 areetched isotropically. In this manner, the end portions of the masks 438are not aligned with end portions and the depression of thesemiconductor layer 403 and end portions of the n⁺ layers 404 a and 404b and the source and drain electrode layers 405 a and 405 b, and theseend portions further recede, so that the shapes of the end portions havecurvature.

In addition, since the etching rates of the end portions of thesemiconductor layer 403, the n⁺ layers 404 a and 404 b, and the sourceand drain electrode layers 405 a and 405 b are different depending onthe etching conditions or oxide semiconductor materials and conductivematerials, the curvatures are different and the end portions are notcontinuous in some cases.

Furthermore, the etchant after the wet etching is removed together withthe etched materials by cleaning. Waste liquid of the etchant containingthe removed materials may be purified to recycle the materials containedin the waste liquid. Materials such as indium contained in the oxidesemiconductor layer are collected from the waste liquid after theetching and recycled, so that resources can be effectively used and costcan be reduced.

After that, the masks 438 are removed.

The material of the source and drain electrode layers 405 a and 405 bpreferably has a higher etching rate than that of the semiconductorlayer 403. This is because, in the case of etching the source and drainelectrode layers 405 a and 405 b and the semiconductor layer 403 in onetime by etching, decreasing the etching rate of the semiconductor layer403 so as to be lower than that of the source and drain electrode layers405 a and 405 b can suppress the excessive etching of the semiconductorlayer 403. As a result, the removal of the semiconductor layer 403 canbe suppressed.

After that, thermal treatment at 200° C. to 600° C., typically 300° C.to 500° C. is preferably performed. Here, thermal treatment is performedat 350° C. for an hour in a nitrogen atmosphere. Through this thermaltreatment, rearrangement at the atomic level of the In—Ga—Zn—O-basedoxide semiconductor used for the semiconductor layer 403 and the n⁺layers 404 a and 404 b occurs. This thermal treatment (includingphoto-annealing or the like) is important in that the distortion thatinterrupts carrier transport in the semiconductor layer 403 and the n⁺layers 404 a and 404 b can be released. Note that there is no particularlimitation on when to perform the thermal treatment, as long as it isperformed after the first oxide semiconductor film 431 and the secondoxide semiconductor film 432 are formed.

In addition, oxygen radical treatment may be performed on the exposeddepression of the semiconductor layer 403. By the oxygen radicaltreatment, the thin film transistor in which the channel formationregion is formed using the semiconductor layer 403 can serve as anormally-off transistor. Moreover, by the radical treatment, the damageof the semiconductor layer 403 due to the etching can be repaired. Theradical treatment is preferably performed in an atmosphere of O₂, N₂O,or an atmosphere of oxygen containing N₂, He, Ar, or the like.Alternatively, an atmosphere obtained by adding Cl₂ or CF₄ to the aboveatmosphere may be used. Note that the radical treatment is preferablyperformed with no bias voltage applied to the substrate 100 side.

Through the above steps, the inverted staggered thin film transistor 420shown in FIG. 1E can be completed.

With the use of the resist mask having regions with a plurality of(typically two kinds of) thicknesses, which is formed using themulti-tone mask, as in this embodiment, the number of resist masks canbe reduced; therefore, the process can be simplified and cost reductioncan be achieved. Accordingly, a reliable semiconductor device can bemanufactured at low cost with high productivity.

Embodiment 2

Here, an example of a semiconductor device including a thin filmtransistor with a structure where the source and drain electrode layersare in contact with the semiconductor layer in Embodiment 1 is describedwith reference to FIGS. 3A to 3E and FIGS. 4A1 and 4A2.

FIG. 4A1 is a plan view of a thin film transistor 460 in a semiconductordevice of this embodiment, and FIG. 4A2 is a cross-sectional view takenalong D1-D2 of FIG. 4A1. The thin film transistor 460 is an invertedstaggered thin film transistor and includes a gate electrode layer 451,a gate insulating layer 452, a semiconductor layer 453, and source anddrain electrode layers 455 a and 455 b.

FIGS. 3A to 3E are cross-sectional views showing steps of manufacturingthe thin film transistor 460.

In FIG. 3A, an insulating film 457 serving as a base film is providedover a substrate 450 and the gate electrode layer 451 is provided overthe insulating film 457. In this embodiment, a silicon oxide film (witha thickness of 100 nm) is used as the insulating film 457. The gateinsulating layer 452, an oxide semiconductor film 481, and a conductivefilm 483 are stacked in that order over the gate electrode layer 451.

A region where the oxide semiconductor film 481 and the conductive film483 are in contact with each other is preferably modified by plasmatreatment. In this embodiment, plasma treatment is performed on theoxide semiconductor film 481 (an In—Ga—Zn—O-based non-single-crystalfilm in this embodiment) in an argon atmosphere before the conductivefilm 483 is formed.

The plasma treatment may be performed using nitrogen, helium, or thelike instead of the argon atmosphere. Alternatively, an argon atmosphereto which oxygen, hydrogen, N₂O, or the like is added may be used.Further alternatively, an argon atmosphere to which Cl₂, CF₄, or thelike is added may be used.

The gate insulating layer 452, the oxide semiconductor film 481, and theconductive film 483 can be formed successively without exposure to air.By the successive formation without exposure to air, the films can bestacked without the interface therebetween contaminated by anatmospheric component or a contaminant impurity element floating in air;therefore, variation in characteristics of a thin film transistor can bedecreased.

A mask 484 is formed over the gate insulating layer 452, the oxidesemiconductor film 481, and the conductive film 483.

In this embodiment, an example is described in which light-exposure isperformed using a multi-tone (high-tone) mask in order to form the mask484. The mask 484 can be formed in a manner similar to that of the mask434 of Embodiment 1.

The light exposure is performed using the multi-tone mask through whichlight is transmitted so as to have a plurality of intensities, and thendevelopment is performed, whereby the mask 484 having regions withdifferent thicknesses can be formed as shown in FIG. 3B. By using amulti-tone mask, the number of light-exposure masks can be reduced.

Next, a first etching step is performed using the mask 484; accordingly,the oxide semiconductor film 481 and the conductive film 483 are etchedinto island shapes. As a result, an oxide semiconductor layer 485 and aconductive layer 487 can be formed (see FIG. 3B).

In this embodiment, the first etching step is performed by dry etchingin which an etching gas is used.

As the etching gas, a gas including chlorine (chlorine-based gas such aschlorine (Cl₂), boron chloride (BCl₃), silicon chloride (SiCl₄), orcarbon tetrachloride (CCl₄)) is preferably used. With the use of the gasincluding chlorine in etching, in-plane variation in etching can bereduced as compared to the case of using a gas without chlorine.

Alternatively, a gas including fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur fluoride (SF₆), nitrogen fluoride(NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂);any of these gases to which a rare gas such as helium (He) or argon (Ar)is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. The etching condition (the amount of electric power applied to acoil-shaped electrode, the amount of electric power applied to anelectrode on a substrate side, the temperature of the electrode on thesubstrate side, or the like) is adjusted as appropriate so that thefilms can be etched into desired shapes.

In this embodiment, an ICP etching method is employed and the etchingcondition is as follows: Cl₂ and O₂ are used; the amount of electricpower applied to the coil-shaped electrode is 1500 W; the amount ofelectric power applied to the electrode on the substrate side is 200 W;a pressure is 1.5 Pa; and a substrate temperature is −10° C.

When the etching is performed using a chlorine-based gas (Cl₂) to whichan oxygen gas (O₂) is added (preferably, the content of oxygen in theetching gas is set to be 15 vol % or more), in the case of using asilicon oxynitride film as the gate insulating layer 452, theselectivity ratio of the In—Ga—Zn—O-based non-single-crystal film usedfor the oxide semiconductor layer 485 with respect to the gateinsulating layer 452 can be increased. Therefore, the oxidesemiconductor film 481 can be etched more than the gate insulating layer452.

Through the first etching step in which the oxide semiconductor film 481and the conductive film 483 are dry-etched, the oxide semiconductor film481 and the conductive film 483 are etched anisotropically. In thismanner, the end portion of the mask 484 is aligned with end portions ofthe oxide semiconductor layer 485 and the conductive layer 487, andthese end portions become continuous.

In addition, since the etching rates of the end portions of the oxidesemiconductor layer 485 and the conductive layer 487 are differentdepending on the etching conditions or, oxide semiconductor materialsand conductive materials, the tapered angles are different and the endportions are not continuous in some cases.

Next, the mask 484 is subjected to ashing. As a result, the mask isreduced in size and thickness. Through the ashing, a region of theresist mask, which has small thickness (a region overlapping with partof the gate electrode layer 451), is removed, so that divided masks 488can be formed (see FIG. 3C).

A second etching step is performed using the masks 488; accordingly, theoxide semiconductor layer 485 and the conductive layer 487 are etchedinto a semiconductor layer 453 and source and drain electrode layers 455a and 455 b (see FIG. 3D). Note that the semiconductor layer 453 ispartly etched to become a semiconductor layer having a groove (adepression) and also having an end portion which is partly etched andexposed.

In this embodiment, the second etching step is performed by wet etchingin which an etchant is used.

As the etchant, a mixed solution of phosphoric acid, acetic acid, andnitric acid, an ammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2), or the like can be used. Alternatively,ITO07N (manufactured by Kanto Chemical Co., Inc) may be used.

The etching condition (etchant, etching time, temperature, or the like)is adjusted as appropriate, depending on a material used for theconductive layer 487, so that the films can be etched into desiredshapes.

For example, in the case where an aluminum film or an aluminum alloyfilm is used for the conductive layer 487, wet etching using a mixedsolution of phosphoric acid, acetic acid, and nitric acid can beperformed. Further, in the case where a titanium film is used for theconductive layer 487, wet etching using an ammonia hydrogen peroxidemixture (hydrogen peroxide:ammonia:water=5:2:2) as the etchant can beperformed.

For example, in the case where an aluminum film or an aluminum alloyfilm is used for the conductive layer 487, the oxide semiconductor layer485 and the conductive layer 487 may be etched using a mixed solution ofphosphoric acid, acetic acid, and nitric acid as the etchant of thefirst etching step.

In the second etching step, the conductive layer and the oxidesemiconductor layer may be etched using different etchants.

For example, in the case where a titanium film is used for theconductive layer 487, the conductive layer 487 is etched using anammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2) as the etchant of the second etching step,and the oxide semiconductor layer 485 may be etched using a mixedsolution of phosphoric acid, acetic acid, and nitric acid.

In the second etching step, the oxide semiconductor layer 485 and theconductive layer 487 may be etched using an ammonia hydrogen peroxidemixture (hydrogen peroxide:ammonia:water=5:2:2) so that a depression ofthe semiconductor layer 453 and the source and drain electrode layers455 a and 455 b can be formed.

In a manner similar to the above, through the second etching step inwhich the oxide semiconductor layer 485 and the conductive layer 487 arewet-etched, the oxide semiconductor layer 485 and the conductive layer487 are etched isotropically. In this manner, the end portions of themasks 488 are not aligned with end portions and the depression of thesemiconductor layer 453 and end portions of the source and drainelectrode layers 455 a and 455 b, and these end portions further recede,so that the shapes of the end portions have curvature.

In addition, since the etching rates of the end portions of thesemiconductor layer 453 and the source and drain electrode layers 455 aand 455 b are different depending on the etching conditions or, oxidesemiconductor materials and conductive materials, the tapered angles aredifferent and the end portions are not continuous in some cases.

Furthermore, the etchant after the wet etching is removed together withthe etched materials by cleaning. Waste liquid of the etchant containingthe removed materials may be purified to recycle the materials containedin the waste liquid. Materials such as indium contained in the oxidesemiconductor layer are collected from the waste liquid after theetching and recycled, so that resources can be effectively used and costcan be reduced.

After that, the masks 488 are removed.

Through the above steps, the inverted staggered thin film transistor 460shown in FIG. 3E can be completed.

With the use of the resist mask having a plurality of (typically twokinds of) thicknesses, which is formed using a multi-tone mask, as inthis embodiment, the number of resist masks can be reduced; therefore,the process can be simplified and cost reduction can be achieved.Accordingly, a reliable semiconductor device can be manufactured at lowcost with high productivity.

Embodiment 3

In this embodiment, a process for manufacturing a display deviceincluding a thin film transistor is described with reference to FIGS. 5Ato 5C, FIGS. 6A to 6C, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIGS. 11A1,11A2, 11B1, and 11B2, and FIG. 12.

As for a substrate 100 having a light-transmitting property shown inFIG. 5A, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like which is typified by #7059 glass,#1737 glass, or the like manufactured by Corning, Inc. can be used.

Next, a conductive layer is formed entirely over a surface of thesubstrate 100, and then a first photolithography process is performed toform a resist mask. Then, an unnecessary portion is removed by etching,so that wirings and electrodes (a gate wiring including a gate electrodelayer 101, a capacitor wiring 108, and a first terminal 121) are formed.At this time, the etching is performed so that at least an end portionof the gate electrode layer 101 is tapered. FIG. 5A is a cross-sectionalview showing this state. Note that FIG. 7 corresponds to a top view ofthis state.

Each of the gate wiring including the gate electrode layer 101, thecapacitor wiring 108, and the first terminal 121 at a terminal portionis preferably formed using a heat-resistant conductive material such asan element selected from titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), or scandium (Sc); analloy including any of these elements; an alloy film including any ofthese elements in combination; or a nitride including any of theseelements. In the case of using a low-resistant conductive material suchas aluminum (Al) or copper (Cu), the low-resistant conductive materialis used in combination with the above heat-resistant conductive materialbecause Al alone has problems of low heat resistance, tendency tocorrode, and the like.

Next, a gate insulating layer 102 is formed entirely over the gateelectrode layer 101. The gate insulating layer 102 is formed to athickness of 50 nm to 250 nm by a sputtering method or the like.

For example, a silicon oxide film is formed to a thickness of 100 nm asthe gate insulating layer 102 by a sputtering method. Needless to say,the gate insulating layer 102 is not limited to such a silicon oxidefilm and another insulating film such as a silicon oxynitride film, asilicon nitride film, an aluminum oxide film, or a tantalum oxide filmmay be formed to have a single-layer or stacked-layer structure.

Note that reverse sputtering in which an argon gas is introduced andplasma is generated is preferably performed before the formation of theoxide semiconductor film, in order to remove dust on the surface of thegate insulating layer. Nitrogen, helium, or the like may be used insteadof the argon atmosphere. Alternatively, oxygen, hydrogen, N₂O, or thelike may be added to the argon atmosphere. Further alternatively, Cl₂,CF₄, or the like may be added to the argon atmosphere.

Next, a first oxide semiconductor film 109 (a first In—Ga—Zn—O-basednon-single-crystal film in this embodiment) is formed over the gateinsulating layer 102. It is effective to deposit the firstIn—Ga—Zn—O-based non-single-crystal film without exposure to air afterthe plasma treatment because dust and moisture do not adhere to theinterface between the gate insulating layer and the semiconductor film.Here, the first In—Ga—Zn—O-based non-single-crystal film is formed underthe following condition: the target is an oxide semiconductor targetincluding In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1) with a diameter of 8inches; the distance between the substrate and the target is 170 mm; apressure is 0.4 Pa; a direct current (DC) power supply is 0.5 kW; and anatmosphere is argon or oxygen. A pulse direct current (DC) power supplyis preferable because dust can be reduced and film thickness becomesuniform. The thickness of the first In—Ga—Zn—O-based non-single-crystalfilm is set in the range of 5 nm to 200 nm. In this embodiment, thethickness of the first In—Ga—Zn—O-based non-single-crystal film is 100nm.

Next, a second oxide semiconductor film 111 (a second In—Ga—Zn—O-basednon-single-crystal film in this embodiment) is formed without exposureto air by a sputtering method. Here, sputtering deposition is performedunder the following condition: the target is In₂O₃:Ga₂O₃:ZnO=1:1:1; apressure is 0.4 Pa; the amount of electric power is 500 W; a depositiontemperature is room temperature; and an argon gas flow rate is 40 sccm.Although the target of In₂O₃:Ga₂O₃:ZnO=1:1:1 is used intentionally, anIn—Ga—Zn—O-based non-single-crystal film including a crystal grain whichhas a size of 1 nm to 10 nm just after the deposition is obtained insome cases. By adjusting a target composition ratio, a depositionpressure (0.1 Pa to 2.0 Pa), the amount of electric power (250 W to 3000W: 8 inchesφ), a temperature (room temperature to 100° C.), a depositioncondition of reactive sputtering, or the like as appropriate, thepresence or absence of the crystal grains and the density of the crystalgrains can be controlled and the diameter of the crystal grain can beadjusted within the range of 1 nm to 10 nm. The thickness of the secondIn—Ga—Zn—O-based non-single-crystal film is 5 nm to 20 nm. Needless tosay, in the case where the film includes the crystal grain, the size ofthe crystal grain does not exceed the film thickness. In thisembodiment, the thickness of the second In—Ga—Zn—O-basednon-single-crystal film is 5 nm.

The first In—Ga—Zn—O-based non-single-crystal film and the secondIn—Ga—Zn—O-based non-single-crystal film are formed under differentconditions from each other. For example, the flow rate ratio of anoxygen gas to an argon gas under the deposition conditions of the firstIn—Ga—Zn—O-based non-single-crystal film is higher than that under thedeposition conditions of the second In—Ga—Zn—O-based non-single-crystalfilm. Specifically, the second In—Ga—Zn—O-based non-single-crystal filmis formed in a rare gas (such as argon or helium) atmosphere (or anatmosphere including an oxygen gas for 10% or less and an argon gas for90% or more), and the first In—Ga—Zn—O-based non-single-crystal film isformed in an oxygen atmosphere (or in an argon gas-to-oxygen gas flowratio of 1 to 1 or higher).

The second In—Ga—Zn—O-based non-single-crystal film may be formed in thechamber where reverse sputtering has been performed previously, or in adifferent chamber from the chamber where reverse sputtering has beenperformed previously.

As the sputtering method, there are an RF sputtering method in which ahigh-frequency power source is used as a sputtering power source, a DCsputtering method, and a pulse DC sputtering method by which bias isapplied in a pulsed manner. The RF sputtering method is used mainly inthe case of forming an insulating film, and the DC sputtering method isused mainly in the case of forming a metal film.

Moreover, there is a multi-source sputtering apparatus in which aplurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside a chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge.

In addition, as a deposition method by a sputtering method, there arealso a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringdeposition to form a thin film of a compound thereof, and a biassputtering method in which voltage is also applied to a substrate duringfilm formation.

Next, a conductive film 132 is formed of a metal material over the firstoxide semiconductor film 109 and the second oxide semiconductor film 111by a sputtering method or a vacuum evaporation method. FIG. 5B is across-sectional view showing this state.

As the material of the conductive film 132, there are an elementselected from Al, Cr, Ta, Ti, Mo, or W, an alloy including any of theseelements, an alloy film including any of the above elements incombination, and the like. In the case of performing thermal treatmentat 200° C. to 600° C., the conductive film 132 is preferably formed soas to resist such thermal treatment. In the case of using Al, Al is usedin combination with a heat-resistant conductive material because Alalone has problems of low heat resistance, tendency to corrode, and thelike. As the heat-resistant conductive material used in combination withAl, an element selected from titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), or scandium (Sc); analloy including any of these elements; an alloy film including any ofthese elements in combination; or a nitride including any of theseelements is used.

Here, the conductive film 132 is a titanium film of a single-layerstructure. Alternatively, the conductive film 132 may have a two-layerstructure; for example, a titanium film is stacked over an aluminumfilm. Further alternatively, the conductive film 132 may have athree-layer structure; for example, a Ti film is formed, an aluminumfilm including Nd (Al—Nd film) is stacked over the Ti film, and a Tifilm is further formed thereover. The conductive film 132 may be analuminum film including silicon of a single-layer structure.

Next, a second photolithography process is performed to form a mask 133which is a resist mask. In this embodiment, an example is described inwhich light exposure is performed using a multi-tone (high-tone) maskfor forming the mask 133. The mask 133 can be formed in a manner similarto that of the mask 434 of Embodiment 1.

The light exposure is performed using the multi-tone mask through whichlight is transmitted so as to have a plurality of intensities, and thendevelopment is performed, whereby the mask 133 including regions withdifferent thicknesses can be formed as shown in FIG. 5C. Accordingly,with the use of a multi-tone mask, the number of light-exposure maskscan be reduced.

Next, a first etching step is performed using the mask 133; accordingly,the first oxide semiconductor film 109 which is the firstIn—Ga—Zn—O-based non-single-crystal film, the second oxide semiconductorfilm 111 which is the second In—Ga—Zn—O-based non-single-crystal film,and the conductive film 132 are etched into island shapes. Accordingly,a first oxide semiconductor layer 134, a second oxide semiconductorlayer 135, and a conductive layer 136 can be formed (see FIG. 5C). FIG.8 is a top view showing this state.

In this embodiment, the first etching step is performed by dry etchingin which an etching gas is used.

As the etching gas, a gas including chlorine (chlorine-based gas such aschlorine (Cl₂), boron chloride (BCl₃), silicon chloride (SiCl₄), orcarbon tetrachloride (CCl₄)) is preferably used. With the use of the gasincluding chlorine in etching, in-plane variation in etching can bereduced as compared to the case of using a gas without chlorine.

Alternatively, a gas including fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur fluoride (SF₆), nitrogen fluoride(NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂);any of these gases to which a rare gas such as helium (He) or argon (Ar)is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to be able to etch the films into desired shapes, theetching condition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

In this embodiment, an ICP etching method is employed and the etchingcondition is as follows: Cl₂ and O₂ are used; the amount of electricpower applied to the coil-shaped electrode is 1500 W; the amount ofelectric power applied to the electrode on the substrate side is 200 W;a pressure is 1.5 Pa; and a substrate temperature is −10° C.

When the etching is performed using a chlorine-based gas (Cl₂) to whichan oxygen gas (O₂) is added (preferably, the content of oxygen in theetching gas is set to be 15 vol % or more), in the case of using asilicon oxynitride film as the gate insulating layer 102, theselectivity ratio of the In—Ga—Zn—O-based non-single-crystal film usedfor the first oxide semiconductor layer 134 and the second oxidesemiconductor layer 135 with respect to the gate insulating layer 102can be increased. Therefore, the oxide semiconductor layers can beetched more than the gate insulating layer 102.

By the first etching step in which the first oxide semiconductor film109, the second oxide semiconductor film 111, and the conductive film132 are dry-etched, the first oxide semiconductor film 109, the secondoxide semiconductor film 111, and the conductive film 132 are etchedanisotropically. In this manner, the end portion of the mask 133 isaligned with end portions of the first oxide semiconductor layer 134,the second oxide semiconductor layer 135, and the conductive layer 136,and these end portions become continuous.

Next, the mask 133 is subjected to ashing. As a result, the mask isreduced in size and thickness. Through the ashing, the region of theresist mask, which has small thickness (region overlapping with part ofthe gate electrode layer 101), is removed, so that divided masks 131 canbe formed (see FIG. 3C).

A second etching step is performed using the masks 131; accordingly, thefirst oxide semiconductor layer 134, the second oxide semiconductorlayer 135, and the conductive layer 136 are etched into a semiconductorlayer 103, n⁺ layers 104 a and 104 b which are source and drain regions,and source and drain electrode layers 105 a and 105 b. Note that thesemiconductor layer 103 is partly etched to become a semiconductor layerhaving a groove (a depression) and also having an end portion which ispartly etched and exposed.

In this embodiment, the second etching step is performed by wet etchingin which an etchant is used.

As the etchant, a mixed solution of phosphoric acid, acetic acid, andnitric acid, an ammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2), or the like can be used.

The etching condition (etchant, etching time, temperature, or the like)is adjusted as appropriate, depending on a material used for theconductive film 132, so that the films can be etched into desiredshapes.

For example, in the case where an aluminum film or an aluminum alloyfilm is used for the conductive layer 136, wet etching using a mixedsolution of phosphoric acid, acetic acid, and nitric acid can beperformed. Further, in the case where a titanium film is used for theconductive layer 136, wet etching using an ammonia hydrogen peroxidemixture (hydrogen peroxide:ammonia:water=5:2:2) as the etchant can beperformed.

For example, in the case where an aluminum film or an aluminum alloyfilm is used for the conductive layer 136, the first oxide semiconductorlayer 134, the second oxide semiconductor layer 135, and the conductivelayer 136 may be etched using a mixed solution of phosphoric acid,acetic acid, and nitric acid as the etchant of the second etching step.

In the second etching step, the conductive layer and the oxidesemiconductor layers may be etched using different etchants.

For example, in the case where a titanium film is used for theconductive layer 136, the conductive layer 136 is etched using anammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2) as the etchant of the second etching step,and the first oxide semiconductor layer 134 and the second oxidesemiconductor layer 135 may be etched using a mixed solution ofphosphoric acid, acetic acid, and nitric acid.

In the second etching step, the first oxide semiconductor layer 134, thesecond oxide semiconductor layer 135, and the conductive layer 136 maybe etched using an ammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2) so that a depression of the semiconductorlayer 103, the n+ layers 104 a and 104 b, and the source and drainelectrode layers 105 a and 105 b can be formed.

Through the second etching step in which the first oxide semiconductorlayer 134, the second oxide semiconductor layer 135, and the conductivelayer 136 are wet-etched, the first oxide semiconductor layer 134, thesecond oxide semiconductor layer 135, and the conductive layer 136 areetched isotropically. In this manner, the end portions and thedepression of the semiconductor layer 103 and the end portions of the n⁺layers 104 a and 104 b and the source and drain electrode layers 105 aand 105 b are not aligned with the end portions of the masks 131 andfurther recede, so that the shapes of the end portions have curvature.

Furthermore, the etchant after the wet etching is removed together withthe etched materials by cleaning. Waste liquid of the etchant containingthe removed materials may be purified to recycle the materials containedin the waste liquid. Materials such as indium contained in the oxidesemiconductor layer are collected from the waste liquid after theetching and recycled, so that resources can be effectively used and costcan be reduced.

Next, thermal treatment at 200° C. to 600° C., typically 300° C. to 500°C., is preferably performed. Here, thermal treatment is performed at350° C. for an hour in a nitrogen atmosphere in a furnace. Through thisthermal treatment, rearrangement at the atomic level of theIn—Ga—Zn—O-based non-single-crystal film occurs. This thermal treatment(also including photo-annealing) is important in that the distortionthat interrupts carrier transport can be released. Note that there is noparticular limitation on the timing of the thermal treatment, as long asit is performed after the second In—Ga—Zn—O-based non-single-crystalfilm is formed. For example, the thermal treatment may be performedafter a pixel electrode is formed.

Further, oxygen radical treatment may be performed on the exposedchannel formation region of the semiconductor layer 103. By the oxygenradical treatment, the thin film transistor can serve as a normally-offtransistor. Moreover, by the radical treatment, the damage of thesemiconductor layer 103 due to the etching can be repaired. The radicaltreatment is preferably performed in an atmosphere of O₂, N₂O, or anatmosphere of N₂, He, Ar, or the like which contains oxygen.Alternatively, an atmosphere obtained by adding Cl₂ or CF₄ to the aboveatmosphere may be used. Note that the radical treatment is preferablyperformed with no bias applied.

Through the above steps, a thin film transistor 170, the channelformation region of which is formed using the semiconductor layer 103,can be completed. FIG. 6A is a cross-sectional view showing this state.Note that FIG. 9 corresponds to the top view of this state.

The second etching step is performed so that a terminal layer 124 formedusing the same material as the semiconductor layer 103, a terminal 123formed using the same material as the n⁺ layers 104 a and 104 b, and asecond terminal 122 formed using the same material as the source anddrain electrode layers 105 a and 105 b are left in a terminal portion.Note that the second terminal 122 is electrically connected to a sourcewiring (a source wiring including the source and drain electrode layers105 a and 105 b).

With the use of the resist mask having a plurality of (typically twokinds of) thicknesses, which is formed using a multi-tone mask, thenumber of resist masks can be reduced; therefore, the process can besimplified and cost reduction can be achieved.

Next, the mask 131 is removed, and a protective insulating layer 107 isformed so as to cover the thin film transistor 170. The protectiveinsulating layer 107 can be formed using a silicon nitride film, asilicon oxide film, a silicon oxynitride film, an aluminum oxide film, atantalum oxide film, or the like which is obtained by a sputteringmethod or the like.

Next, a third photolithography process is performed to form a resistmask. The gate insulating layer 102 and the protective insulating layer107 are etched to form a contact hole 125 that reaches the drainelectrode layer 105 b. Moreover, by this etching, a contact hole 127that reaches the second terminal 122 and a contact hole 126 that reachesthe first terminal 121 are also formed. FIG. 6B is a cross-sectionalview showing this state.

Next, the resist mask is removed and then a transparent conductive filmis formed. The transparent conductive film is formed using indium oxide(In₂O₃), indium oxide-tin oxide alloy (In₂O₃—SnO₂, abbreviated as ITO),or the like by a sputtering method, a vacuum evaporation method, or thelike. Films of these materials are etched using a solution includinghydrochloric acid. However, since etching of ITO particularly tends toleave residue, indium oxide-zinc oxide alloy (In₂O₃—ZnO) may be used inorder to improve etching processability.

Next, a fourth photolithography process is performed to form a resistmask. An unnecessary portion is removed by etching, whereby a pixelelectrode layer 110 is formed.

Moreover, by this fourth photolithography process, the capacitor wiring108 and the pixel electrode layer 110 together form a storage capacitorby using, as a dielectric, the gate insulating layer 102 and theprotective insulating layer 107 in a capacitor portion.

Furthermore, in the fourth photolithography process, the first terminaland the second terminal are covered with the resist mask. Accordingly,transparent conductive films 128 and 129 formed in the terminal portionsare left. The transparent conductive films 128 and 129 each serve as anelectrode or a wiring used for connection to an FPC. The transparentconductive film 128 formed over the first terminal 121 is used for aterminal electrode used for connection which serves as an input terminalof the gate wiring. The transparent conductive film 129 formed over thesecond terminal 122 is used for a terminal electrode used for connectionwhich serves as an input terminal of the source wiring.

Next, the resist mask is removed. FIG. 6C is a cross-sectional viewshowing this state. Note that FIG. 10 corresponds to a top view of thisstate.

FIGS. 11A1 and 11A2, respectively, are a cross-sectional view and a topview and show the gate wiring terminal portion in this state. FIG. 11A1corresponds to a cross-sectional view taken along E1-E2 of FIG. 11A2. InFIG. 11A1, a transparent conductive film 155 formed over the protectiveinsulating film 154 is used for a terminal electrode for connectionwhich serves as an input terminal. In the terminal portion of FIG. 11A1,a first terminal 151 formed using the same material as the gate wiringoverlaps with a connection electrode layer 153 formed using the samematerial as the source wiring with a gate insulating layer 152, asemiconductor layer 157, and an n⁺ layer 158 interposed therebetween,and the first terminal 151 and the connection electrode layer 153 arebrought into conduction via the transparent conductive film 155. Notethat the portion where the transparent conductive film 128 is in contactwith the first terminal 121 in FIG. 6C corresponds to a portion wherethe transparent conductive film 155 is in contact with the firstterminal 151 in FIG. 11A1.

FIGS. 11B1 and 11B2, respectively, are a cross-sectional view and a topview and show a source wiring terminal portion which is different fromthe source wiring terminal portion of FIG. 6C. Moreover, FIG. 11B1corresponds to a cross-sectional view taken along D1-D2 of FIG. 11B2. InFIG. 11B1, the transparent conductive film 155 formed over theprotective insulating film 154 is used for a terminal electrode forconnection which serves as an input terminal. In the terminal portion ofFIG. 11B1, an electrode layer 156 formed using the same material as thegate wiring is disposed under a second terminal 150 with the gateinsulating layer 152 interposed therebetween. The electrode layer 156 isnot electrically connected to the second terminal 150, and a capacitoras a counter measure against noise or static electricity can be formedby setting the potential of the electrode layer 156 so as to bedifferent from that of the second terminal 150, for example, floating,GND, 0 V, or the like. The second terminal 150 is electrically connectedto the transparent conductive film 155 via the protective insulatingfilm 154.

A plurality of gate wirings, source wirings, and capacitor wirings areprovided in accordance with the pixel density. In the terminal portion,a plurality of terminals are arranged: the first terminal having thesame potential as the gate wiring; the second terminal having the samepotential as the source wiring; a third terminal having the samepotential as the capacitor wiring; and the like. The numbers ofrespective terminals may be determined as appropriate by a practitioner.

Through the four photolithography processes performed in this manner,the storage capacitor and the pixel thin film transistor portionincluding the thin film transistor 170 which is a bottom-gate n-channelthin film transistor can be completed by using four photomasks. Then,they are arranged in matrix corresponding to pixels, so that a pixelportion is formed; thus, one substrate for use in manufacturing anactive matrix display device is obtained. In this specification, such asubstrate is called an active matrix substrate for convenience.

In the case of manufacturing an active matrix liquid crystal displaydevice, a liquid crystal layer is provided between an active matrixsubstrate and a counter substrate which is provided with a counterelectrode and then the active matrix substrate and the counter substrateare fixed to each other. Note that a common electrode which iselectrically connected to the counter electrode of the counter substrateis provided over the active matrix substrate and a fourth terminal whichis electrically connected to the common electrode is provided in theterminal portion. The fourth terminal is used for setting the potentialof the common electrode to be fixed, for example GND, 0 V, or the like.

The pixel structure is not limited to the pixel structure shown in FIG.10. FIG. 12 shows an example of a top view which is different from thatin FIG. 10. In the example shown in FIG. 12, the capacitor wiring is notprovided and a storage capacitor is formed in such a manner that thepixel electrode overlaps with the gate wiring of the adjacent pixel withthe protective insulating film and the gate insulating layer interposedtherebetween; in this case, the capacitor wiring and the third terminalconnected to the capacitor wiring can be omitted. Note that in FIG. 12,the same portion as in FIG. 10 is denoted by the same reference numeral.

In an active matrix liquid crystal display device, a display pattern isformed on a screen by driving the pixel electrodes arranged in matrix.Specifically, the liquid crystal layer provided between the pixelelectrode and the counter electrode is optically modulated by applyingvoltage between the selected pixel electrode and the counter electrodecorresponding to the selected pixel electrode, and this opticalmodulation is recognized as a display pattern by an observer.

In the display of motion pictures by a liquid crystal display device,the response speed of a liquid crystal molecule itself is slow.Therefore, there are problems of afterimages or blur of motion pictures.In order to improve the characteristics of a liquid crystal displaydevice regarding motion pictures, there is a driving technique by whichblack display on the entire screen is performed every one frame, whichis so-called black insertion.

Moreover, there is a driving technique by which the normal verticalcycle is increased 1.5 times or 2 times or more in order to improve theresponse speed and the grayscale to be written is selected for everyplural fields which have been divided in each frame, which is so-calleddouble frame rate driving.

Moreover, in order to improve the characteristics of a liquid crystaldisplay device regarding motion pictures, there is a driving techniqueby which a plane light source is formed using a plurality of LEDs(light-emitting diodes), a plurality of EL light sources, or the likeand each light source of the plane light source is used independently toperform intermittent lighting driving within one frame period. As theplane light source, three or more kinds of LEDs may be used or an LEDthat emits white light may be used. Since the plurality of LEDs can becontrolled independently, the light emission timing of the LEDs can besynchronized with the timing at which the optical modulation of theliquid crystal layer is switched. By this driving technique, the LED canbe partly turned off; therefore, particularly in the case of displayingan image having a large part black display regions in one screen,reduction in power consumption is achieved.

With the use of any of these driving techniques in combination, thedisplay characteristics of a liquid crystal display device, such as thecharacteristic in displaying motion pictures, can be improved ascompared to those of conventional liquid crystal display devices.

In the n-channel transistor obtained in this embodiment, the channelformation region is formed using the In—Ga—Zn—O-based non-single-crystalfilm and this transistor has favorable operating characteristics.Therefore, these driving techniques can be used in combination.

In the case of manufacturing a light-emitting display device, thepotential of one electrode of an organic light-emitting element (alsocalled a cathode) is set to a low power supply potential, for example toGND, 0 V, or the like. Therefore, a terminal portion is provided with afourth terminal for setting the potential of the cathode to a low powersupply potential, for example to GND, 0 V, or the like. Moreover, in thecase of manufacturing a light-emitting display device, a power supplyline is provided in addition to the source wiring and the gate wiring.Therefore, the terminal portion is provided with a fifth terminalelectrically connected to the power supply line.

As described in this embodiment, the use of the oxide semiconductor forthe thin film transistor leads to reduction in manufacturing cost.

As in this embodiment, with the use of the resist mask having aplurality of (typically two kinds of) thicknesses, which is formed usinga multi-tone mask, the number of resist masks can be reduced; therefore,the process can be simplified and cost reduction can be achieved.Accordingly, a reliable semiconductor device can be manufactured at lowcost with high productivity.

This embodiment can be implemented in combination with any of thestructures disclosed in the other embodiments, as appropriate.

Embodiment 4

In this embodiment, an example of manufacturing at least part of adriver circuit and a thin film transistor of a pixel portion over onesubstrate in a display device which is an example of a semiconductordevice will be described below.

The thin film transistor in the pixel portion is formed in accordancewith any of Embodiments 1 to 3. The thin film transistor described inany of Embodiments 1 to 3 is an n-channel TFT; therefore, part of adriver circuit which can be formed using an n-channel TFT is formed overthe same substrate as the thin film transistor of the pixel portion.

FIG. 14A shows an example of a block diagram of an active matrix liquidcrystal display device which is an example of a semiconductor device.The display device shown in FIG. 14A includes, over a substrate 5300, apixel portion 5301 having a plurality of pixels that is each providedwith a display element; a scanning-line driver circuit 5302 that selectseach pixel; and a signal-line driver circuit 5303 that controls a videosignal input to a selected pixel.

The pixel portion 5301 is connected to the signal-line driver circuit5303 with a plurality of signal lines S1 to Sm (not shown) extending ina column direction from the signal-line driver circuit 5303 andconnected to the scanning-line driver circuit 5302 with a plurality ofscanning lines G1 to Gn (not shown) extending in a row direction fromthe scanning-line driver circuit 5302. The pixel portion 5301 includes aplurality of pixels (not shown) arranged in matrix corresponding to thesignal lines S1 to Sm and the scanning lines G1 to Gn. In addition, eachof the pixels is connected to a signal line Sj (any one of the signallines S1 to Sm) and a scanning line Gi (any one of the scanning lines G1to Gn).

The thin film transistor described in any of Embodiments 1 to 3 is ann-channel TFT, and a signal-line driver circuit including an n-channelTFT is described with reference to FIG. 15.

The signal-line driver circuit shown in FIG. 15 includes a driver IC5601, switch groups 5602_1 to 5602_M, a first wiring 5611, a secondwiring 5612, a third wiring 5613, and wirings 5621_1 to 5621_M. Each ofthe switch groups 5602_1 to 5602_M includes a first thin film transistor5603 a, a second thin film transistor 5603 b, and a third thin filmtransistor 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M.Each of the switch groups 5602_1 to 5602_M is connected to the firstwiring 5611, the second wiring 5612, the third wiring 5613, and one ofthe wirings 5621_1 to 5621_M corresponding to the switch groups 5602_1to 5602_M, respectively. Each of the wirings 5621_1 to 5621_M isconnected to three signal lines through the first thin film transistor5603 a, the second thin film transistor 5603 b, and the third thin filmtransistor 5603 c. For example, a wiring 5621_J of the J-th column (anyone of the wirings 5621_1 to 5621_M) is connected to a signal line Sj−1,a signal line Sj, and a signal line Sj+1 through the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c of a switch group 5602_J.

Note that a signal is inputted to each of the first wiring 5611, thesecond wiring 5612, and the third wiring 5613.

Note that the driver IC 5601 is preferably formed on a single crystalsubstrate. Further, the switch groups 5602_1 to 5602_M are preferablyformed over the same substrate as the pixel portion. Therefore, thedriver IC 5601 is preferably connected to the switch groups 5602_1 to5602_M through an FPC or the like.

Next, operation of the signal-line driver circuit shown in FIG. 15 isdescribed with reference to a timing chart of FIG. 16. FIG. 16 shows thetiming chart where the scanning line Gi in the i-th row is selected.Further, a selection period of the scanning line Gi in the i-th row isdivided into a first sub-selection period T1, a second sub-selectionperiod T2, and a third sub-selection period T3. Furthermore, thesignal-line driver circuit in FIG. 15 operates similarly to that in FIG.16 even when a scanning line of another row is selected.

Note that the timing chart of FIG. 16 shows the case where the wiring5621_J in the J-th column is connected to the signal line Sj−1, thesignal line Sj, and the signal line Sj+1 through the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c.

The timing chart of FIG. 16 shows timing when the scanning line Gi inthe i-th row is selected, timing 5703 a of on/off of the first thin filmtransistor 5603 a, timing 5703 b of on/off of the second thin filmtransistor 5603 b, timing 5703 c of on/off of the third thin filmtransistor 5603 c, and a signal 5721_J inputted to the wiring 5621_J inthe J-th column.

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinputted to the wirings 5621_1 to 5621_M. For example, a video signalinputted to the wiring 5621_J in the first sub-selection period T1 isinputted to the signal line Sj−1, a video signal inputted to the wiring5621_J in the second sub-selection period T2 is inputted to the signalline Sj, and a video signal inputted to the wiring 5621_J in the thirdsub-selection period T3 is inputted to the signal line Sj+1. The videosignals inputted to the wiring 5621_J in the first sub-selection periodT1, the second sub-selection period T2, and the third sub-selectionperiod T3 are denoted by Data_j−1, Data_j, and Data_j+1, respectively.

As shown in FIG. 16, in the first sub-selection period T1, the firstthin film transistor 5603 a is turned on, and the second thin filmtransistor 5603 b and the third thin film transistor 5603 c are turnedoff. At this time, Data_j−1 inputted to the wiring 5621_J is inputted tothe signal line Sj−1 through the first thin film transistor 5603 a. Inthe second sub-selection period T2, the second thin film transistor 5603b is turned on, and the first thin film transistor 5603 a and the thirdthin film transistor 5603 c are turned off. At this time, Data_jinputted to the wiring 5621J is inputted to the signal line Sj throughthe second thin film transistor 5603 b. In the third sub-selectionperiod T3, the third thin film transistor 5603 c is turned on, and thefirst thin film transistor 5603 a and the second thin film transistor5603 b are turned off. At this time, Data_j+1 inputted to the wiring5621_J is inputted to the signal line Sj+1 through the third thin filmtransistor 5603 c.

As described above, in the signal-line driver circuit of FIG. 15, onegate selection period is divided into three; thus, video signals can beinputted to three signal lines through one wiring 5621 in one gateselection period. Therefore, in the signal-line driver circuit of FIG.15, the number of connections between the substrate provided with thedriver IC 5601 and the substrate provided with the pixel portion can bereduced to approximately one third of the number of signal lines. Whenthe number of connections is reduced to approximately one third of thenumber of signal lines, the reliability, yield, and the like of thesignal-line driver circuit of FIG. 15 can be improved.

Note that there is no particular limitation on the arrangement, number,driving method, and the like of the thin film transistor as long as onegate selection period is divided into a plurality of sub-selectionperiods and video signals are inputted to a plurality of signal linesfrom one wiring in each of the plurality of sub-selection periods, asshown in FIG. 15.

For example, when video signals are inputted to three or more signallines from one wiring in each of three or more sub-selection periods, athin film transistor and a wiring for controlling the thin filmtransistor may be added. Note that when one gate selection period isdivided into four or more sub-selection periods, one sub-selectionperiod becomes shorter. Therefore, one gate selection period ispreferably divided into two or three sub-selection periods.

As another example, as shown in a timing chart of FIG. 17, one selectionperiod may be divided into a pre-charge period Tp, the firstsub-selection period T1, the second sub-selection period T2, and thethird sub-selection period T3. Further, the timing chart of FIG. 17shows timing when the scanning line Gi in the i-th row is selected,timing 5803 a of on/off of the first thin film transistor 5603 a, timing5803 b of on/off of the second thin film transistor 5603 b, timing 5803c of on/off of the third thin film transistor 5603 c, and a signal5821_J inputted to the wiring 5621_J in the J-th column. As shown inFIG. 17, the first thin film transistor 5603 a, the second thin filmtransistor 5603 b, and the third thin film transistor 5603 c are turnedon in the pre-charge period Tp. At this time, a pre-charge voltage V_(p)inputted to the wiring 5621_J is inputted to the signal line Sj−1, thesignal line Sj, and the signal line Sj+1 through the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c, respectively. In the first sub-selectionperiod T1, the first thin film transistor 5603 a is turned on, and thesecond thin film transistor 5603 b and the third thin film transistor5603 c are turned off. At this time, Data_j−1 inputted to the wiring5621_J is inputted to the signal line Sj−1 through the first thin filmtransistor 5603 a. In the second sub-selection period T2, the secondthin film transistor 5603 b is turned on, and the first thin filmtransistor 5603 a and the third thin film transistor 5603 c are turnedoff. At this time, Data_j inputted to the wiring 5621J is inputted tothe signal line Sj through the second thin film transistor 5603 b. Inthe third sub-selection period T3, the third thin film transistor 5603 cis turned on, and the first thin film transistor 5603 a and the secondthin film transistor 5603 b are turned off. At this time, Data_j+1inputted to the wiring 5621_J is inputted to the signal line Sj+1through the third thin film transistor 5603 c.

As described above, in the signal-line driver circuit of FIG. 15, towhich the timing chart of FIG. 17 is applied, a signal line can bepre-charged by providing a pre-charge selection period beforesub-selection periods. Thus, a video signal can be written to a pixel athigh speed. Note that portions in FIG. 17 similar to those in FIG. 16are denoted by the same reference numerals, and detailed description ofthe same portions and portions having similar functions is omitted.

In addition, a configuration of the scanning-line driver circuit isdescribed. The scanning-line driver circuit includes a shift registerand a buffer. Moreover, a level shifter may be included in some cases.In the scanning-line driver circuit, when a clock signal (CLK) and astart pulse signal (SP) are inputted to the shift register, a selectionsignal is generated. The generated selection signal is buffered andamplified by the buffer, and the resulting signal is supplied to acorresponding scanning line. Gate electrodes of transistors in pixelscorresponding to one line are connected to the scanning line. Further,since the transistors in the pixels of one line have to be turned on atthe same time, a buffer which can feed a large amount of current is usedfor the buffer.

One mode of the shift register used for part of the scanning-line drivercircuit is described with reference to FIG. 18 and FIG. 19.

FIG. 18 shows a circuit configuration of the shift register. The shiftregister shown in FIG. 18 includes a plurality of flip-flops, flip-flops5701_1 to 5701 _(—) n. Further, the shift register operates by inputtinga first clock signal, a second clock signal, a start pulse signal, and areset signal.

Connection relationships of the shift register in FIG. 18 are described.In a flip-flop 5701 _(—) i (any one of the flip-flops 5701_1 to 5701_(—) n) in an i-th stage of the shift register in FIG. 18, a firstwiring 5501 shown in FIG. 19 is connected to a seventh wiring 5717 _(—)i−1; a second wiring 5502 shown in FIG. 19 is connected to a seventhwiring 5717 _(—) i+1; a third wiring 5503 shown in FIG. 19 is connectedto a seventh wiring 5717 _(—) i; and a sixth wiring 5506 shown in FIG.19 is connected to a fifth wiring 5715.

Further, a fourth wiring 5504 shown in FIG. 19 is connected to a secondwiring 5712 in a flip-flop in an odd-numbered stage, and is connected toa third wiring 5713 in a flip-flop of an even-numbered stage. A fifthwiring 5505 shown in FIG. 19 is connected to a fourth wiring 5714.

Note that the first wiring 5501 shown in FIG. 19 of the flip-flop 5701_1of a first stage is connected to a first wiring 5711, and the secondwiring 5502 shown in FIG. 19 of the flip-flop 5701 _(—) n of an n-thstage is connected to a sixth wiring 5716.

The first wiring 5711, the second wiring 5712, the third wiring 5713,and the sixth wiring 5716 may be called a first signal line, a secondsignal line, a third signal line, and a fourth signal line,respectively. Further, the fourth wiring 5714 and the fifth wiring 5715may be called a first power supply line and a second power supply line,respectively.

Next, FIG. 19 shows details of the flip-flop shown in FIG. 18. Theflip-flop shown in FIG. 19 includes a first thin film transistor 5571, asecond thin film transistor 5572, a third thin film transistor 5573, afourth thin film transistor 5574, a fifth thin film transistor 5575, asixth thin film transistor 5576, a seventh thin film transistor 5577,and an eighth thin film transistor 5578. Note that the first thin filmtransistor 5571, the second thin film transistor 5572, the third thinfilm transistor 5573, the fourth thin film transistor 5574, the fifththin film transistor 5575, the sixth thin film transistor 5576, theseventh thin film transistor 5577, and the eighth thin film transistor5578 are n-channel transistors, and are brought into an on state when avoltage between a gate and a source (V_(gs)) exceeds a threshold voltage(V_(th)).

Next, a connection structure of the flip-flop shown in FIG. 19 isdescribed below.

A first electrode (one of a source electrode and a drain electrode) ofthe first thin film transistor 5571 is connected to the fourth wiring5504, and a second electrode (the other of the source electrode and thedrain electrode) of the first thin film transistor 5571 is connected tothe third wiring 5503.

A first electrode of the second thin film transistor 5572 is connectedto the sixth wiring 5506. A second electrode of the second thin filmtransistor 5572 is connected to the third wiring 5503.

A first electrode of the third thin film transistor 5573 is connected tothe fifth wiring 5505. A second electrode of the third thin filmtransistor 5573 is connected to a gate electrode of the second thin filmtransistor 5572. A gate electrode of the third thin film transistor 5573is connected to the fifth wiring 5505.

A first electrode of the fourth thin film transistor 5574 is connectedto the sixth wiring 5506. A second electrode of the fourth thin filmtransistor 5574 is connected to the gate electrode of the second thinfilm transistor 5572. A gate electrode of the fourth thin filmtransistor 5574 is connected to a gate electrode of the first thin filmtransistor 5571.

A first electrode of the fifth thin film transistor 5575 is connected tothe fifth wiring 5505. A second electrode of the fifth thin filmtransistor 5575 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the fifth thin film transistor5575 is connected to the first wiring 5501.

A first electrode of the sixth thin film transistor 5576 is connected tothe sixth wiring 5506. A second electrode of the sixth thin filmtransistor 5576 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the sixth thin film transistor5576 is connected to the gate electrode of the second thin filmtransistor 5572.

A first electrode of the seventh thin film transistor 5577 is connectedto the sixth wiring 5506. A second electrode of the seventh thin filmtransistor 5577 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the seventh thin filmtransistor 5577 is connected to the second wiring 5502. A firstelectrode of the eighth thin film transistor 5578 is connected to thesixth wiring 5506. A second electrode of the eighth thin film transistor5578 is connected to the gate electrode of the second thin filmtransistor 5572. A gate electrode of the eighth thin film transistor5578 is connected to the first wiring 5501.

Note that the point at which the gate electrode of the first thin filmtransistor 5571, the gate electrode of the fourth thin film transistor5574, the second electrode of the fifth thin film transistor 5575, thesecond electrode of the sixth thin film transistor 5576, and the secondelectrode of the seventh thin film transistor 5577 are connected isreferred to as a node 5543. Further, the point at which the gateelectrode of the second thin film transistor 5572, the second electrodeof the third thin film transistor 5573, the second electrode of thefourth thin film transistor 5574, the gate electrode of the sixth thinfilm transistor 5576, and the second electrode of the eighth thin filmtransistor 5578 are connected is referred to as a node 5544.

The first wiring 5501, the second wiring 5502, the third wiring 5503,and the fourth wiring 5504 may be called a first signal line, a secondsignal line, a third signal line, and a fourth signal line,respectively. The fifth wiring 5505 and the sixth wiring 5506 may becalled a first power supply line and a second power supply line,respectively.

Alternatively, the signal-line driver circuit and the scanning-linedriver circuit can be manufactured using only the n-channel TFTsdescribed in Embodiment 1. Since the n-channel TFTs described inEmbodiment 1 have high mobility, the driving frequency of the drivercircuits can be increased. In addition, parasitic capacitance of then-channel TFTs described in Embodiment 1 is reduced because of sourceand drain regions which are formed using an In—Ga—Zn—O-basednon-single-crystal film; therefore, the frequency characteristics (whichare called f characteristics) of the n-channel TFTs are high. Forexample, the scanning-line driver circuit including the n-channel TFTsdescribed in Embodiment 1 can operate at high speed; therefore, it ispossible to increase the frame frequency or to achieve insertion of ablack screen, for example.

In addition, when the channel width of the transistor in thescanning-line driver circuit is increased or a plurality ofscanning-line driver circuits are provided, for example, much higherframe frequency can be realized. When a plurality of scanning-linedriver circuits are provided, a scanning-line driver circuit for drivingeven-numbered scanning lines is provided on one side and a scanning-linedriver circuit for driving odd-numbered scan lines is provided on theopposite side; thus, increase in frame frequency can be realized. Inaddition, it is advantageous for increase in size of a display device tooutput a signal to the same scanning line from the plurality ofscanning-line driver circuits.

In the case of manufacturing an active matrix light-emitting displaydevice which is an example of a semiconductor device, a plurality ofscanning-line driver circuits are preferably arranged because aplurality of thin film transistors are arranged in at least one pixel.An example of a block diagram of an active matrix light-emitting displaydevice is shown in FIG. 14B.

The light-emitting display device shown in FIG. 14B includes, over asubstrate 5400, a pixel portion 5401 having a plurality of pixels eachprovided with a display element; a first scanning-line driver circuit5402 and a second scanning-line driver circuit 5404 that select eachpixel; and a signal-line driver circuit 5403 that controls a videosignal input to a selected pixel.

In the case of inputting a digital video signal to the pixel of thelight-emitting display device shown in FIG. 14B, the pixel is put in alight-emitting state or a non-light-emitting state by switching on/offof a transistor. Thus, grayscale can be displayed using an area-ratiograyscale method or a time-ratio grayscale method. An area-ratiograyscale method refers to a driving method by which one pixel isdivided into a plurality of sub-pixels and the respective sub-pixels aredriven separately based on video signals so that grayscale is displayed.Further, a time-ratio grayscale method refers to a driving method bywhich a period during which a pixel is in a light-emitting state iscontrolled so that grayscale is displayed.

Since the response time of light-emitting elements is shorter than thatof liquid crystal elements or the like, the light-emitting elements aresuitable for a time-ratio grayscale method. Specifically, in the case ofdisplaying with a time gray scale method, one frame period is dividedinto a plurality of sub-frame periods. Then, in accordance with videosignals, the light-emitting element in the pixel is put in alight-emitting state or a non-light-emitting state in each sub-frameperiod. By dividing a frame into a plurality of sub-frames, the totallength of time, in which pixels actually emit light in one frame period,can be controlled with video signals to display gray scales.

Note that in the light-emitting display device shown in FIG. 14B, in thecase where one pixel includes two switching TFTs, a signal which isinputted to a first scanning line serving as a gate wiring of one of theswitching TFTs is generated from the first scanning-line driver circuit5402 and a signal which is inputted to a second scanning line serving asa gate wiring of the other of the switching TFTs is generated from thesecond scanning-line driver circuit 5404. However, the signal which isinputted to the first scanning line and the signal which is inputted tothe second scanning line may be generated together from onescanning-line driver circuit. In addition, for example, there is apossibility that a plurality of the scanning lines used for controllingthe operation of the switching element be provided in each pixeldepending on the number of switching TFTs included in one pixel. In thiscase, the signals which are inputted to the plurality of scanning linesmay be generated all from one scanning-line driver circuit or may begenerated from a plurality of scanning-line driver circuits.

Even in the light-emitting display device, part of the driver circuitwhich can be formed using the n-channel TFTs can be provided over thesame substrate as the thin film transistors of the pixel portion.Moreover, the signal-line driver circuit and the scanning-line drivercircuit can be manufactured using only the n-channel TFTs described inany of Embodiments 1 to 3.

The aforementioned driver circuits may be used for not only a liquidcrystal display device or a light-emitting display device but alsoelectronic paper in which electronic ink is driven by utilizing anelement electrically connected to a switching element. The electronicpaper is also called an electrophoretic display device (electrophoreticdisplay) and has advantages in that it has the same level of readabilityas regular paper, it has less power consumption than other displaydevices, and it can be set to have a thin and light form.

There is a variety of modes of electrophoretic displays. Theelectrophoretic display is a device in which a plurality ofmicrocapsules each including first particles having positive charge andsecond particles having negative charge are dispersed in a solvent or asolute, and an electrical field is applied to the microcapsules so thatthe particles in the microcapsules move in opposite directions from eachother, and only a color of the particles gathered on one side isdisplayed. Note that the first particles or the second particles includea colorant, and does not move when there is no electric field. Inaddition, a color of the first particles is different from a color ofthe second particles (the particles may also be colorless).

Thus, the electrophoretic display utilizes a so-called dielectrophoreticeffect in which a substance with high dielectric constant moves to aregion with high electric field. The electrophoretic display does notrequire a polarizing plate and a counter substrate, which are necessaryfor a liquid crystal display device, so that the thickness and weightthereof are about half.

That which the microcapsules are dispersed in a solvent is calledelectronic ink, and this electronic ink can be printed on a surface ofglass, plastic, fabric, paper, or the like. Color display is alsopossible with the use of a color filter or particles including acoloring matter.

In addition, an active matrix display device can be completed byproviding, as appropriate, a plurality of the microcapsules over anactive matrix substrate so as to be interposed between two electrodes,and can perform display by application of electric field to themicrocapsules. For example, the active matrix substrate obtained usingthe thin film transistor of any of Embodiments 1 to 3 can be used.

Note that the first particles and the second particles in themicrocapsule may be formed from one of a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, and a magnetophoretic material, ora composite material thereof.

Through the above process, a highly reliable light-emitting displaydevice as a semiconductor device can be manufactured.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments, as appropriate.

Embodiment 5

A thin film transistor can be manufactured, and the thin film transistorcan be used for a pixel portion and further for a driver circuit, sothat a semiconductor device having a display function (also referred toas a display device) can be manufactured. Moreover, a thin filmtransistor can be used for part of a driver circuit or an entire drivercircuit formed over the same substrate as a pixel portion, so that asystem-on-panel can be formed.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. A light-emitting elementincludes, in its scope, an element whose luminance is controlled bycurrent or voltage, and specifically includes an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

In addition, the display device includes a panel in which a displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel. Regarding one mode of an elementsubstrate before the display element is completed in a process formanufacturing the display device, the element substrate is provided witha unit which can supply current to the display element in each of aplurality of pixels. Specifically, the element substrate may be in astate provided with only a pixel electrode of the display element, astate after a conductive film to be a pixel electrode is formed andbefore the conductive film is etched to form the pixel electrode, or anyother states.

A display device in this specification refers to an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device includes any of the followingmodules in its category: a module including a connector such as anflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP); a module having a TAB tape or a TCP whichis provided with a printed wiring board at the end thereof; and a modulehaving an integrated circuit (IC) which is directly mounted on a displayelement by a chip-on-glass (COG) method.

The appearance and a cross section of a liquid crystal display panelwhich is one mode of a semiconductor device will be described in thisembodiment with reference to FIGS. 22A1 and 22A2, and FIG. 22B. FIGS.22A1 and 22A2 are top views of a panel in each of which highly reliablethin film transistors 4010 and 4011 that include semiconductor layers ofthe In—Ga—Zn—O-based non-single-crystal films described in Embodiment 1and a liquid crystal element 4013, which are formed over a firstsubstrate 4001, are sealed with a sealant 4005 between the firstsubstrate 4001 and a second substrate 4006. FIG. 22B corresponds to across-sectional view of FIGS. 22A1 and 22A2 along line M-N.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scanning-line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scanning-line driver circuit 4004. Thus, the pixelportion 4002 and the scanning-line driver circuit 4004 as well as aliquid crystal layer 4008 are sealed with the sealant 4005 between thefirst substrate 4001 and the second substrate 4006. A signal-line drivercircuit 4003 which is formed using a single crystal semiconductor filmor a polycrystalline semiconductor film over a substrate which isprepared separately is mounted in a region that is different from theregion surrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on a connection method ofthe driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 22A1 showsan example in which the signal-line driver circuit 4003 is mounted by aCOG method, and FIG. 22A2 shows an example in which the signal-linedriver circuit 4003 is mounted by a TAB method.

Each of the pixel portion 4002 and the scanning-line driver circuit 4004which are provided over the first substrate 4001 includes a plurality ofthin film transistors. FIG. 22B shows the thin film transistor 4010included in the pixel portion 4002 and the thin film transistor 4011included in the scanning-line driver circuit 4004. Insulating layers4020 and 4021 are provided over the thin film transistors 4010 and 4011.

As each of the thin film transistors 4010 and 4011, the highly reliablethin film transistor shown in Embodiment 3 including theIn—Ga—Zn—O-based non-single-crystal film as the semiconductor layer canbe used. Alternatively, the thin film transistor described in Embodiment1 or 2 may be applied. In this embodiment, the thin film transistors4010 and 4011 are each an n-channel thin film transistor.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode layer 4030,the counter electrode layer 4031, and the liquid crystal layer 4008overlap with each other corresponds to the liquid crystal element 4013.Note that the pixel electrode layer 4030 and the counter electrode layer4031 are provided with an insulating layer 4032 and an insulating layer4033 serving as orientation films, respectively, and the liquid crystallayer 4008 is interposed between the insulating layers 4032 and 4033.

Note that the first substrate 4001 and the second substrate 4006 can beformed from glass, metal (typically, stainless steel), ceramic, orplastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. Alternatively, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films can beused.

A columnar spacer 4035 which is formed by etching an insulating filmselectively is provided to control a distance (a cell gap) between thepixel electrode layer 4030 and the counter electrode layer 4031.Alternatively, a spherical spacer may be used. In addition, the counterelectrode layer 4031 is electrically connected to a common potentialline provided over the same substrate as the thin film transistor 4010.The counter electrode layer 4031 and the common potential line areelectrically connected to each other through conductive particles whichare arranged between the pair of substrates using a common connectionportion. Note that the conductive particles are contained in the sealant4005.

Alternatively, a blue phase liquid crystal without an orientation filmmay be used. A blue phase is a type of liquid crystal phase whichappears just before a cholesteric liquid crystal changes into anisotropic phase when the temperature of the cholesteric liquid crystalis increased. A blue phase appears only within narrow temperature range;therefore, the liquid crystal layer 4008 is formed using a liquidcrystal composition in which a chiral agent of 5 wt. % or more is mixedin order to expand the temperature range. The liquid crystal compositionincluding a blue phase liquid crystal and a chiral agent has a shortresponse time of 10 μs to 100 μs and is optically isotropic; therefore,orientation treatment is not necessary and viewing angle dependence issmall.

Note that this embodiment describes an example of a transmissive liquidcrystal display device; however, the present invention can be applied toa reflective liquid crystal display device or a semi-transmissive liquidcrystal display device.

Although a liquid crystal display device of this embodiment has apolarizer provided outer than the substrate (the viewer side) and acolor layer and an electrode layer of a display element provided innerthan the substrate, which are arranged in that order, the polarizer maybe inner than the substrate. The stacked structure of the polarizer andthe color layer is not limited to that shown in this embodiment and maybe set as appropriate in accordance with the materials of the polarizerand the color layer and the condition of the manufacturing process.Further, a light-blocking film serving as a black matrix may beprovided.

In this embodiment, in order to reduce the unevenness of the surface ofthe thin film transistors and to improve the reliability of the thinfilm transistors, the thin film transistors which are obtained inEmbodiment 3 are covered with protective films or insulating layers (theinsulating layers 4020 and 4021) serving as planarizing insulatingfilms. Note that the protective film is provided to prevent entry of acontaminant impurity such as an organic substance, a metal substance, ormoisture floating in the atmosphere, and therefore a dense film ispreferable. The protective film may be formed using a single layer or astack of layers of a silicon oxide film, a silicon nitride film, asilicon oxynitride film, a silicon nitride oxide film, an aluminum oxidefilm, an aluminum nitride film, an aluminum oxynitride film, or analuminum nitride oxide film. Although the protective film is formed by asputtering method in this embodiment, the method is not particularlylimited and may be selected from a variety of methods.

Here, the insulating layer 4020 is formed to have a stacked structure asthe protective film. Here, a silicon oxide film is formed by asputtering method as a first layer of the insulating layer 4020. The useof a silicon oxide film for the protective film provides an advantageouseffect of preventing hillock of an aluminum film used for a sourceelectrode layer and a drain electrode layer.

Moreover, an insulating layer is formed as a second layer of theprotective film. Here, a silicon nitride film is formed by a sputteringmethod as a second layer of the insulating layer 4020. When a siliconnitride film is used for the protective film, it is possible to preventmovable ions such as sodium from entering a semiconductor region to varythe electrical characteristics of the TFT.

Further, after the protective film is formed, the semiconductor layermay be annealed (at 300° C. to 400° C.).

Further, the insulating layer 4021 is formed as the planarizinginsulating film. The insulating layer 4021 can be formed from an organicmaterial having heat resistance, such as polyimide, acrylic,benzocyclobutene, polyamide, or epoxy. As an alternative to such organicmaterials, it is possible to use a low-dielectric constant material (alow-k material), a siloxane-based resin, PSG (phosphosilicate glass),BPSG (borophosphosilicate glass), or the like. Note that the insulatinglayer 4021 may be formed by stacking a plurality of insulating filmsformed of these materials.

Note that a siloxane-based resin is a resin formed from a siloxane-basedmaterial as a starting material and having the bond of Si—O—Si. Thesiloxane-based resin may include as a substituent an organic group (forexample, an alkyl group or an aryl group) or a fluoro group.Alternatively, the organic group may include a fluoro group.

The method for the formation of the insulating layer 4021 is notparticularly limited and any of the following methods can be useddepending on the material of the insulating layer 4021: a sputteringmethod, an SOG method, spin coating, dip coating, spray coating, adroplet discharge method (e.g., an inkjet method, screen printing, oroffset printing), a doctor knife, a roll coater, a curtain coater, aknife coater, or the like. In the case of forming the insulating layer4021 with the use of a material solution, annealing (300° C. to 400° C.)may be performed on the semiconductor layer at the same time as a bakingstep. When the baking of the insulating layer 4021 and the annealing ofthe semiconductor layer are performed at the same time, a semiconductordevice can be manufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed from a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

A conductive composition including a conductive macromolecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed using the conductive composition has preferably a sheetresistance of 10000 ohm/square or less and a transmittance of 70% ormore at a wavelength of 550 nm. Further, the resistivity of theconductive high molecule included in the conductive composition ispreferably 0.1 Ω·cm or less.

As the conductive macromolecule, a so-called π-electron conjugatedconductive macromolecule can be used. As examples thereof, polyanilineor a derivative thereof, polypyrrole or a derivative thereof,polythiophene or a derivative thereof, a copolymer of two or more kindsof them, and the like can be given.

Further, a variety of signals and potentials are supplied from an FPC4018 to the signal-line driver circuit 4003 which is formed separately,the scanning-line driver circuit 4004, and the pixel portion 4002.

In this embodiment, a connecting terminal electrode 4015 is formed usingthe same conductive film as the pixel electrode layer 4030 included inthe liquid crystal element 4013. A terminal electrode 4016 is formedusing the same conductive film as the source and drain electrode layersincluded in the thin film transistors 4010 and 4011. Note that theconnection terminal electrode 4015 and the terminal electrode 4016 areformed over an n+ layer 4025 and a semiconductor layer 4026.

The connecting terminal electrode 4015 is electrically connected to aterminal of the FPC 4018 through an anisotropic conductive film 4019.

Although FIGS. 22A1, 22A2, and 22B show an example in which thesignal-line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001, this embodiment is not limited to this structure.The scanning-line driver circuit may be separately formed and thenmounted, or only part of the signal-line driver circuit or part of thescanning-line driver circuit may be separately formed and then mounted.

FIG. 23 shows an example in which a liquid crystal display module isformed as a semiconductor device using a TFT substrate 2600 which ismanufactured according to the manufacturing method disclosed in thisspecification.

FIG. 23 shows an example of a liquid crystal display module, in whichthe TFT substrate 2600 and a counter substrate 2601 are fixed to eachother with a sealant 2602, and a pixel portion 2603 including a TFT andthe like, a display element 2604 including a liquid crystal layer, and acolor layer 2605 are provided between the substrates to form a displayregion. The color layer 2605 is necessary to perform color display. Inthe case of the RGB system, respective colored layers corresponding tocolors of red, green, and blue are provided for respective pixels.Polarizing plates 2606 and 2607 and a diffuser plate 2613 are providedoutside the TFT substrate 2600 and the counter substrate 2601. A lightsource includes a cold cathode tube 2610 and a reflective plate 2611. Acircuit board 2612 is connected to a wiring circuit portion 2608 of theTFT substrate 2600 through a flexible wiring board 2609 and includes anexternal circuit such as a control circuit and a power source circuit.The polarizing plate and the liquid crystal layer may be stacked with aretardation plate interposed therebetween.

For the liquid crystal display module, a TN (Twisted Nematic) mode, anIPS (In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, anMVA (Multi-domain Vertical Alignment) mode, a PVA (Patterned VerticalAlignment) mode, an ASM (Axially Symmetric aligned Micro-cell) mode, anOCB (Optical Compensated Birefringence) mode, an FLC (FenoelectricLiquid Crystal) mode, an AFLC (AntiFerroelectric Liquid Crystal) mode,or the like can be used.

Through the above process, a highly reliable liquid crystal displaypanel as a semiconductor device can be manufactured.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments, as appropriate.

Embodiment 6

In this embodiment, an example of electronic paper is shown as asemiconductor device.

FIG. 13 shows active matrix electronic paper as an example of asemiconductor device. A thin film transistor 581 used for asemiconductor device, which can be manufactured in a manner similar tothat of the thin film transistor described in Embodiment 3, is a highlyreliable thin film transistor including an In—Ga—Zn—O-basednon-single-crystal film as a semiconductor layer. Alternatively, thethin film transistor described in Embodiment 1 or 2 can be employed asthe thin film transistor 581 described in this embodiment.

The electronic paper in FIG. 13 is an example of a display device inwhich a twisting ball display system is employed. The twisting balldisplay system refers to a method in which spherical particles eachcolored in black and white are arranged between a first electrode layerand a second electrode layer which are electrode layers used for adisplay element, and a potential difference is generated between thefirst electrode layer and the second electrode layer to controlorientation of the spherical particles, so that display is performed.

The thin film transistor 581 formed over a substrate 580 has abottom-gate structure in which the source and drain electrode layer iselectrically connected to a first electrode layer 587 through an openingformed in an insulating layer 583, an insulating layer 584, and aninsulating layer 585. Between the first electrode layer 587 and a secondelectrode layer 588, spherical particles 589 are provided. Eachspherical particle 589 includes a black region 590 a and a white region590 b, and a cavity 594 filled with liquid around the black region 590 aand the white region 590 b. The circumference of the spherical particle589 is filled with filler 595 such as a resin (see FIG. 13). In thisembodiment, the first electrode layer 587 corresponds to a pixelelectrode, and the second electrode layer 588 corresponds to a commonelectrode. The second electrode layer 588 is electrically connected to acommon potential line provided over the same substrate 580 as the thinfilm transistor 581. The second electrode layer 588 and the commonpotential line are electrically connected through conductive particlesarranged between a pair of substrates using the common connectionportion.

Further, instead of the twisting ball, an electrophoretic element can beused. A microcapsule having a diameter of approximately 10 μm to 200 μm,which is filled with transparent liquid, positively-charged whitemicroparticles, and negatively-charged black microparticles, is used. Inthe microcapsule which is provided between the first electrode layer andthe second electrode layer, when an electric field is applied by thefirst electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides toeach other, so that white or black can be displayed. A display elementusing this principle is an electrophoretic display element, and iscalled electronic paper in general. The electrophoretic display elementhas higher reflectance than a liquid crystal display element, and thusan assistant light is unnecessary. Moreover, power consumption is lowand a display portion can be recognized even in a dusky place.Furthermore, an image which is displayed once can be retained even whenpower is not supplied to the display portion. Accordingly, a displayedimage can be stored even though a semiconductor device having a displayfunction (which is also simply referred to as a display device or asemiconductor device provided with a display device) is distanced froman electric wave source.

Through the above process, highly reliable electronic paper as asemiconductor device can be manufactured.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments, as appropriate.

Embodiment 7

This embodiment describes an example of a light-emitting display deviceas a semiconductor device. As an example of a display element of thedisplay device, here, a light-emitting element utilizingelectroluminescence is used. Light-emitting elements utilizingelectroluminescence are classified according to whether a light-emittingmaterial is an organic compound or an inorganic compound. In general,the former is referred to as an organic EL element and the latter isreferred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and thus current flows. Then, those carriers (electrons and holes) arerecombined, and thus the light-emitting organic compound is excited.When the light-emitting organic compound returns to a ground state fromthe excited state, light is emitted. Owing to such a mechanism, such alight-emitting element is referred to as a current-excitationlight-emitting element.

Inorganic EL elements are classified according to their elementstructures into a dispersion type inorganic EL element and a thin-filmtype inorganic EL element. A dispersion type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film type inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade using an organic EL element as a light-emitting element.

FIG. 20 shows an example of a pixel structure to which digital timegrayscale driving can be applied, as an example of a semiconductordevice.

A structure and operation of a pixel to which digital time grayscaledriving can be applied are described. In this example, one pixelincludes two n-channel transistors in each of which a channel formationregion includes an oxide semiconductor layer (In—Ga—Zn—O-basednon-single-crystal film).

A pixel 6400 includes a switching transistor 6401, a driver transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scanning line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driver transistor 6402. The gate of the driver transistor 6402 isconnected to a power supply line 6407 through the capacitor 6403, afirst electrode of the driver transistor 6402 is connected to the powersupply line 6407, and a second electrode of the driver transistor 6402is connected to a first electrode (pixel electrode) of thelight-emitting element 6404. A second electrode of the light-emittingelement 6404 corresponds to a common electrode 6408. The commonelectrode 6408 is electrically connected to a common potential lineformed over one substrate.

The second electrode (common electrode 6408) of the light-emittingelement 6404 is set to a low power supply potential. The low powersupply potential is a potential satisfying the low power supplypotential <a high power supply potential when the high power supplypotential set to the power supply line 6407 is a reference. As the lowpower supply potential, for example, GND, 0 V, or the like may beemployed. A potential difference between the high power supply potentialand the low power supply potential is applied to the light-emittingelement 6404 and current is supplied to the light-emitting element 6404,so that the light-emitting element 6404 emits light. Here, in order tomake the light-emitting element 6404 emit light, each potential is setso that the potential difference between the high power supply potentialand the low power supply potential is greater than or equal to forwardthreshold voltage of the light-emitting element 6404.

Note that gate capacitor of the driver transistor 6402 may be used as asubstitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitance of the driver transistor 6402 may beformed between the channel region and the gate electrode.

In the case of a voltage-input voltage driving method, a video signal isinputted to the gate of the driver transistor 6402 so that the drivertransistor 6402 is in either of two states of being sufficiently turnedon and turned off. That is, the driver transistor 6402 operates in alinear region. In order for the driver transistor 6402 to operate in alinear region, a voltage higher than the voltage of the power supplyline 6407 is applied to the gate of the driver transistor 6402. Notethat a voltage higher than or equal to voltage of the power supplyline+V_(th) of the driver transistor 6402 is applied to the signal line6405.

In the case of performing analog grayscale driving instead of digitaltime grayscale driving, the same pixel structure as that in FIG. 20 canbe used by changing signal input.

In the case of performing analog grayscale driving, a voltage higherthan or equal to forward voltage of the light-emitting element6404+V_(th) of the driver transistor 6402 is applied to the gate of thedriver transistor 6402. The forward voltage of the light-emittingelement 6404 indicates a voltage at which a desired luminance isobtained, and includes at least forward threshold voltage. The videosignal by which the driver transistor 6402 operates in a saturationregion is inputted, so that current can be supplied to thelight-emitting element 6404. In order for the driver transistor 6402 tooperate in a saturation region, the potential of the power supply line6407 is set higher than the gate potential of the driver transistor6402. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 6404 in accordance with the videosignal and perform analog grayscale driving.

Note that the pixel structure shown in FIG. 20 is not limited thereto.For example, a switch, a resistor, a capacitor, a transistor, a logiccircuit, or the like may be added to the pixel shown in FIG. 20.

Next, structures of a light-emitting element are described withreference to FIGS. 21A to 21C. A cross-sectional structure of a pixel isdescribed here by taking an n-channel driver TFT as an example. TFTs7001, 7011, and 7021 serving as driver TFTs used for a semiconductordevice, which are shown in FIGS. 21A, 21B, and 21C, can be manufacturedin a manner similar to that of the thin film transistor described inEmbodiment 3. The TFTs 7001, 7011, and 7021 are highly reliable thinfilm transistors each including an In—Ga—Zn—O-based non-single-crystalfilm as a semiconductor layer. Alternatively, the thin film transistorsdescribed in Embodiment 1 or 2 can be employed as the TFTs 7001, 7011,and 7021.

In order to extract light emitted from the light-emitting element, atleast one of an anode and a cathode may be transparent. A thin filmtransistor and a light-emitting element are formed over a substrate. Alight-emitting element can have a top-emission structure in which lightemission is extracted through the surface opposite to the substrate; abottom-emission structure in which light emission is extracted throughthe surface on the substrate side; or a dual-emission structure in whichlight emission is extracted through the surface opposite to thesubstrate and the surface on the substrate side. The pixel structure canbe applied to a light-emitting element having any of these emissionstructures.

A light-emitting element having a top-emission structure is describedwith reference to FIG. 21A.

FIG. 21A is a cross-sectional view of a pixel in the case where the TFT7001 serving as a driver TFT is an n-channel TFT and light generated ina light-emitting element 7002 is emitted to pass through an anode 7005.In FIG. 21A, a cathode 7003 of the light-emitting element 7002 iselectrically connected to the TFT 7001 serving as a driver TFT, and alight-emitting layer 7004 and the anode 7005 are stacked in this orderover the cathode 7003. The cathode 7003 can be formed using any of avariety of conductive materials as long as it has a low work functionand reflects light. For example, Ca, Al, CaF, MgAg, AlLi, or the like ispreferably used. The light-emitting layer 7004 may be formed using asingle layer or by stacking a plurality of layers. When thelight-emitting layer 7004 is formed by stacking a plurality of layers,the light-emitting layer 7004 is formed by stacking anelectron-injecting layer, an electron-transporting layer, alight-emitting layer, a hole-transporting layer, and a hole-injectinglayer in this order over the cathode 7003. It is not necessary to formall of these layers. The anode 7005 is formed using a light-transmittingconductive material, for example, a light-transmitting conductive filmsuch as a film of indium oxide including tungsten oxide, indium zincoxide including tungsten oxide, indium oxide including titanium oxide,indium tin oxide including titanium oxide, indium tin oxide (hereinafterreferred to as ITO), indium zinc oxide, or indium tin oxide to whichsilicon oxide is added may be used.

The light-emitting element 7002 corresponds to a region where thecathode 7003 and the anode 7005 sandwich the light-emitting layer 7004.In the case of the pixel shown in FIG. 21A, light is emitted from thelight-emitting element 7002 to the anode 7005 side as indicated by anarrow.

Next, a light-emitting element having a bottom-emission structure isdescribed with reference to FIG. 21B. FIG. 21B is a cross-sectional viewof a pixel in the case where the driver TFT 7011 is an n-channel TFT,and light generated in a light-emitting element 7012 is emitted to acathode 7013 side. In FIG. 21B, the cathode 7013 of the light-emittingelement 7012 is formed over a light-transmitting conductive film 7017which is electrically connected to the driver TFT 7011, and alight-emitting layer 7014 and an anode 7015 are stacked in this orderover the cathode 7013. When the anode 7015 has a light-transmittingproperty, a light-blocking film 7016 for reflecting or blocking lightmay be formed so as to cover the anode 7015. As in the case of FIG. 21A,the cathode 7013 can be formed using any of a variety of conductivematerials as long as it has a low work function. Note that the cathode7013 is formed to have a thickness that can transmit light (preferably,approximately 5 nm to 30 nm). For example, an aluminum film with athickness of 20 nm can be used as the cathode 7013. As in the case ofFIG. 21A, the light-emitting layer 7014 may be formed using a singlelayer or by stacking a plurality of layers. As in the case of FIG. 21A,the anode 7015 is not required to transmit light, but can be formedusing a light-transmitting conductive material. For the light-blockingfilm 7016, for example, metal or the like that reflects light can beused; however, the light-blocking film 7016 is not limited to a metalfilm. For example, a resin or the like to which black pigment is addedcan be used.

The light-emitting element 7012 corresponds to a region where thecathode 7013 and the anode 7015 sandwich the light-emitting layer 7014.In the case of the pixel shown in FIG. 21B, light is emitted from thelight-emitting element 7012 to the cathode 7013 side as indicated by anarrow.

Next, a light-emitting element having a dual-emission structure isdescribed with reference to FIG. 21C. In FIG. 21C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driver TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. As in the case of FIG. 21A, thecathode 7023 can be formed using any of a variety of conductivematerials as long as it has a low work function. Note that the cathode7023 is formed to have a thickness that can transmit light. For example,an Al film with a thickness of 20 nm can be used as the cathode 7023. Asin the case of FIG. 21A, the light-emitting layer 7024 may be formedusing a single layer or by stacking a plurality of layers. As in thecase of FIG. 21A, the anode 7025 can be formed using alight-transmitting conductive material.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 overlapwith each other. In the pixel shown in FIG. 21C, light is emitted fromthe light-emitting element 7022 to both the anode 7025 side and thecathode 7023 side as indicated by arrows.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element can be alternatively provided as alight-emitting element.

Note that this embodiment describes the example in which a thin filmtransistor (a driver TFT) which controls the driving of a light-emittingelement is electrically connected to the light-emitting element;however, a structure may be employed in which a current control TFT isconnected between the driver TFT and the light-emitting element.

The semiconductor device described in this embodiment is not limited tothe structures shown in FIGS. 21A to 21C, and can be modified in variousways based on the spirit of techniques disclosed in this specification.

Next, the appearance and cross section of a light-emitting display panel(also referred to as a light-emitting panel) which corresponds to oneembodiment of a semiconductor device is described with reference toFIGS. 24A and 24B. FIG. 24A is a top view of a panel in which a thinfilm transistor and a light-emitting element formed over a firstsubstrate are sealed between the first substrate and a second substratewith a sealant, and FIG. 24B is a cross-sectional view taken along H-Iof FIG. 24A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal-line driver circuits 4503 a and 4503 b, and scanning-line drivercircuits 4504 a and 4504 b, which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal-line driver circuits 4503 a and 4503 b, and thescanning-line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal-line driver circuits 4503 a and 4503 b, and thescanning-line driver circuits 4504 a and 4504 b are sealed together withfiller 4507 by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. In this manner, it is preferable that the pixelportion 4502, the signal-line driver circuits 4503 a and 4503 b, and thescanning-line driver circuits 4504 a and 4504 b be packaged (sealed)with a protective film (such as an attachment film or an ultravioletcurable resin film) or a cover material with high air-tightness andlittle degasification so that the pixel portion 4502, the signal-linedriver circuits 4503 a and 4503 b, and the scanning-line driver circuits4504 a and 4504 b are not exposed to external air.

The pixel portion 4502, the signal-line driver circuits 4503 a and 4503b, and the scanning-line driver circuits 4504 a and 4504 b provided overthe first substrate 4501 each include a plurality of thin filmtransistors, and a thin film transistor 4510 included in the pixelportion 4502 and a thin film transistor 4509 included in the signal-linedriver circuit 4503 a are shown as an example in FIG. 24B.

As the thin film transistors 4509 and 4510, highly reliable thin filmtransistors described in Embodiment 3 including the In—Ga—Zn—O-basednon-single-crystal films as semiconductor layers can be employed.Alternatively, the thin film transistors described in Embodiment 1 or 2may be employed as the thin film transistors 4509 and 4510. In thisembodiment, the thin film transistors 4509 and 4510 are n-channel thinfilm transistors.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to source anddrain electrode layers of the thin film transistor 4510. Note thatalthough the light-emitting element 4511 has a stacked structure of thefirst electrode layer 4517, an electroluminescent layer 4512, and asecond electrode layer 4513, the structure of the light-emitting element4511 is not limited to the structure described in this embodiment. Thestructure of the light-emitting element 4511 can be changed asappropriate depending on a direction in which light is extracted fromthe light-emitting element 4511, or the like.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, or organic polysiloxane. It is particularly preferablethat the partition 4520 be formed using a photosensitive material tohave an opening on the first electrode layer 4517 so that a sidewall ofthe opening is formed as a tilted surface with continuous curvature.

The electroluminescent layer 4512 may be formed using a single layer orby stacking a plurality of layers.

In order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide,or the like into the light-emitting element 4511, a protective film maybe formed over the second electrode layer 4513 and the partition 4520.As the protective film, a silicon nitride film, a silicon nitride oxidefilm, a DLC film, or the like can be formed.

In addition, a variety of signals and potentials are supplied from FPCs4518 a and 4518 b to the signal-line driver circuits 4503 a and 4503 b,the scanning-line driver circuits 4504 a and 4504 b, or the pixelportion 4502.

In this embodiment, a connecting terminal electrode 4515 is formed usingthe same conductive film as the first electrode layer 4517 included inthe light-emitting element 4511. A terminal electrode 4516 is formedusing the same conductive film as the source and drain electrode layersincluded in the thin film transistors 4509 and 4510. Note that theconnection terminal electrode 4515 and the terminal electrode 4516 areformed over an n⁺ layer 4525 and a semiconductor layer 4526.

The connecting terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

The substrate located in the direction in which light is extracted fromthe light-emitting element 4511 needs to have a light-transmittingproperty. In that case, a light-transmitting material such as a glassplate, a plastic plate, a polyester film, or an acrylic film is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin as well as inert gas such as nitrogen or argon can be used. Forexample, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)can be used. In this embodiment, nitrogen is used for the filler 4507.

In addition, if needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retarder plate (a quarter-wave plate, a half-wave plate), or acolor filter may be provided on an emission surface of thelight-emitting element, as appropriate. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment can be performed by whichreflected light is diffused in the depression/projection of the surfaceand glare can be reduced.

As the signal-line driver circuits 4503 a and 4503 b and thescanning-line driver circuits 4504 a and 4504 b, driver circuits formedusing a single crystal semiconductor film or polycrystallinesemiconductor film over a substrate separately prepared may be mounted.In addition, only the signal-line driver circuit or only part thereof,or only the scanning-line driver circuit or only part thereof may beseparately formed and mounted. This embodiment is not limited to thestructure shown in FIGS. 24A and 24B.

Through the above process, a highly reliable light-emitting displaydevice (display panel) as a semiconductor device can be manufactured.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments, as appropriate.

Embodiment 8

A semiconductor device disclosed in this specification can be applied aselectronic paper. Electronic paper can be used for electronic appliancesof every field for displaying information. For example, electronic papercan be used for electronic book (e-book), posters, advertisements invehicles such as trains, display in a variety of cards such as creditcards, and so on. Examples of such electronic appliances are shown inFIGS. 25A and 25B and FIG. 26.

FIG. 25A shows a poster 2631 formed using electronic paper. If theadvertizing medium is printed paper, the advertisement is replaced bymanpower; however, when electronic paper disclosed in this specificationis used, the advertisement display can be changed in a short time.Moreover, a stable image can be obtained without display deterioration.Note that the poster may send and receive information wirelessly.

FIG. 25B shows an advertisement 2632 in a vehicle such as a train. Ifthe advertizing medium is printed paper, the advertisement is replacedby manpower; however, when electronic paper disclosed in thisspecification is used, the advertisement display can be changed in ashort time without much manpower. Moreover, a stable image can beobtained without display deterioration. Note that the advertisement maysend and receive information wirelessly.

FIG. 26 shows an example of an electronic book 2700. For example, theelectronic book 2700 includes two chassis of a chassis 2701 and achassis 2703. The chassis 2701 and 2703 are bound with each other by anaxis portion 2711, along which the electronic book 2700 can be openedand closed. With such a structure, operation as a paper book isachieved.

A display portion 2705 is incorporated in the chassis 2701, and adisplay portion 2707 is incorporated in the chassis 2703. The displayportions 2705 and 2707 may display a series of images, or may displaydifferent images. With the structure where different images aredisplayed in different display portions, for example, the right displayportion (the display portion 2705 in FIG. 26) can display text, and theleft display portion (the display portion 2707 in FIG. 26) can displayimages.

FIG. 26 shows an example in which the chassis 2701 is provided with anoperation portion and the like. For example, the chassis 2701 isprovided with a power supply 2721, an operation key 2723, a speaker2725, and the like. The page can be turned with the operation key 2723.Note that a keyboard, a pointing device, and the like may be provided onthe same plane as the display portion of the chassis. Further, a rearsurface or a side surface of the chassis may be provided with anexternal connection terminal (an earphone terminal, a USB terminal, aterminal which can be connected with a variety of cables such as an ACadopter or a USB cable, and the like), a storage medium insertingportion, or the like. Moreover, the electronic book 2700 may have afunction of an electronic dictionary.

Further, the electronic book 2700 may send and receive informationwirelessly. Desired book data or the like can be purchased anddownloaded from an electronic book server wirelessly.

Embodiment 9

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). As theelectronic appliances, for example, there are a television device (alsoreferred to as TV or a television receiver), a monitor for a computer orthe like, a digital camera, a digital video camera, a digital photoframe, a cellular phone (also referred to as a mobile phone or aportable telephone device), a portable game machine, a portableinformation terminal, an audio playback device, a large game machinesuch as a pachinko machine, and the like.

FIG. 27A shows an example of a television device 9600. A display portion9603 is incorporated in a chassis 9601 of the television device 9600.The display portion 9603 can display images. Here, the chassis 9601 issupported on a stand 9605.

The television device 9600 can be operated by an operation switch of thechassis 9601 or a separate remote controller 9610. The channel andvolume can be controlled with operation keys 9609 of the remotecontroller 9610, and the images displayed in the display portion 9603can be controlled. Moreover, the remote controller 9610 may have adisplay portion 9607 in which the information outgoing from the remotecontroller 9610 is displayed.

Note that the television device 9600 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver, between receivers, or the like) informationcommunication can be performed.

FIG. 27B shows an example of a digital photo frame 9700. For example, adisplay portion 9703 is incorporated in a chassis 9701 of the digitalphoto frame 9700. The display portion 9703 can display a variety ofimages. For example, image data taken by a digital camera or the like isdisplayed, so that the digital photo frame can function in a mannersimilar to that of a general picture frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (such as a USB terminal, aterminal which can be connected to a variety of cables including a USBcable, or the like), a storage medium inserting portion, and the like.These structures may be incorporated on the same plane as the displayportion; however, they are preferably provided on the side surface orrear surface of the display portion because the design is improved. Forexample, a memory including image data taken by a digital camera isinserted into the storage medium inserting portion of the digital photoframe and the image data is imported. Then, the imported image data canbe displayed in the display portion 9703.

The digital photo frame 9700 may send and receive informationwirelessly. In this case, desired image data can be wirelessly importedinto the digital photo frame 9700 and can be displayed therein.

FIG. 28A shows a portable game machine including a chassis 9881 and achassis 9891 which are jointed with a connector 9893 so as to be able toopen and close. A display portion 9882 is incorporated in the chassis9881, and a display portion 9883 is incorporated in the chassis 9891.The portable game machine shown in FIG. 28A additionally includes aspeaker portion 9884, a storage medium inserting portion 9886, an LEDlamp 9890, an input means (operation keys 9885, a connection terminal9887, a sensor 9888 (including a function of measuring force,displacement, position, speed, acceleration, angular speed, the numberof rotations, distance, light, liquid, magnetism, temperature, chemicalsubstance, sound, time, hardness, electric field, current, voltage,electric power, radiation, flow rate, humidity, tilt angle, vibration,smell, or infrared ray), and a microphone 9889), and the like. Needlessto say, the structure of the portable game machine is not limited to theabove, and may be any structure as long as at least a semiconductordevice disclosed in this specification is provided. Moreover, anotheraccessory may be provided as appropriate. The portable game machineshown in FIG. 28A has a function of reading out a program or data storedin a storage medium to display it on the display portion and a functionof sharing information with another portable game machine by wirelesscommunication. The functions of the portable game machine shown in FIG.28A are not limited to these, and the portable game machine can have avariety of functions.

FIG. 28B shows an example of a slot machine 9900 which is a large gamemachine. A display portion 9903 is incorporated in a chassis 9901 of theslot machine 9900. The slot machine 9900 additionally includes anoperation means such as a start lever or a stop switch, a coin slot, aspeaker, and the like. Needless to say, the structure of the slotmachine 9900 is not limited to the above, and may be any structure aslong as at least a semiconductor device disclosed in this specificationis provided. Moreover, another accessory may be provided as appropriate.

FIG. 29A shows an example of a cellular phone 1000. The cellular phone1000 includes a chassis 1001 in which a display portion 1002 isincorporated, and moreover includes an operation button 1003, anexternal connection port 1004, a speaker 1005, a microphone 1006, andthe like.

Information can be inputted to the cellular phone 1000 shown in FIG. 29Aby touching the display portion 1002 with a finger or the like.Moreover, operations such as making a phone call or texting message canbe performed by touching the display portion 1002 with a finger or thelike.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such as text. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are mixed.

For example, in the case of making a phone call or texting message, thedisplay portion 1002 is set to a text input mode where text input ismainly performed, and text input operation can be performed on a screen.In this case, it is preferable to display a keyboard or number buttonson almost the entire screen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 1000, display in the screen of the display portion 1002can be automatically switched by judging the direction of the cellularphone 1000 (whether the cellular phone 1000 is placed horizontally orvertically for a landscape mode or a portrait mode).

Further, the screen modes are switched by touching the display portion1002 or operating the operation button 1003 of the chassis 1001.Alternatively, the screen modes can be switched depending on kinds ofimage displayed in the display portion 1002. For example, when a signalfor an image displayed in the display portion is data of moving images,the screen mode is switched to the display mode. When the signal is textdata, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion1002 is not performed within a specified period while a signal detectedby an optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 1002 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenby touching the display portion 1002 with the palm or the finger,whereby personal authentication can be performed. Moreover, when abacklight which emits near-infrared light or a sensing light sourcewhich emits near-infrared light is provided in the display portion, afinger vein, a palm vein, or the like can be taken.

FIG. 29B shows an example of a cellular phone as well. The cellularphone in FIG. 29B includes a display device 9410 having, in a chassis9411, a display portion 9412 and operation buttons 9413, and acommunication device 9400 having, in a chassis 9401, operation buttons9402, an external input terminal 9403, a microphone 9404, a speaker9405, and a light-emitting portion 9406 which emits light when a phonecall is received. The display device 9410 having a display function canbe detached from or attached to the communication device 9400 having aphone function in the two directions indicated by arrows. Accordingly,the display device 9410 and the communication device 9400 can beattached to each other along respective short axes or long axes.Alternatively, in the case where only the display function is needed,the display device 9410 is detached from the communication device 9400,and then the display device 9410 can be used alone. Images or inputinformation can be transmitted and received by wireless or wirecommunication between the communication device 9400 and the displaydevice 9410, each of which includes a chargeable battery.

This application is based on Japanese Patent Application serial No.2008-274520 filed with Japan Patent Office on Oct. 24, 2008, the entirecontents of which are hereby incorporated by reference.

1. A method for manufacturing a semiconductor device, comprising:forming a gate electrode layer over a substrate having an insulatingsurface; stacking a gate insulating layer, an oxide semiconductor film,and a conductive film over the gate electrode layer; forming a firstmask layer over the gate insulating layer, the oxide semiconductor film,and the conductive film; performing a first etching with the first masklayer to etch the oxide semiconductor film and the conductive film sothat an oxide semiconductor layer and a conductive layer are formed;forming a second mask layer by ashing the first mask layer; andperforming a second etching with the second mask layer to etch the oxidesemiconductor layer and the conductive layer so that an oxidesemiconductor layer having a depression, a source electrode layer, and adrain electrode layer are formed, wherein the first mask layer is formedusing a light-exposure mask, wherein the first etching is dry etchingwith use of an etching gas, wherein the second etching is wet etchingwith use of an etchant, and wherein the oxide semiconductor layer havingthe depression includes a region with a smaller thickness than a regionoverlapping with the source electrode layer or the drain electrodelayer.
 2. The method for manufacturing a semiconductor device accordingto claim 1, wherein the oxide semiconductor film includes indium,gallium, and zinc.
 3. The method for manufacturing a semiconductordevice according to claim 1, wherein the etching gas includes chlorine.4. The method for manufacturing a semiconductor device according toclaim 3, wherein the etching gas further includes oxygen.
 5. The methodfor manufacturing a semiconductor device according to claim 1, whereinan ammonia hydrogen peroxide mixture is used as the etchant in thesecond etching.
 6. The method for manufacturing a semiconductor deviceaccording to claim 1, wherein a half-tone mask or a gray-tone mask isused as the light-exposure mask.
 7. A method for manufacturing asemiconductor device, comprising: forming a gate electrode layer over asubstrate having an insulating surface; stacking a gate insulatinglayer, a first oxide semiconductor film, a second oxide semiconductorfilm, and a conductive film over the gate electrode layer; forming afirst mask layer over the gate insulating layer, the first oxidesemiconductor film, the second oxide semiconductor film, and theconductive film; performing a first etching with the first mask layer toetch the first oxide semiconductor film, the second oxide semiconductorfilm, and the conductive film so that a first oxide semiconductor layer,a second oxide semiconductor layer, and a conductive layer are formed;forming a second mask layer by ashing the first mask layer; andperforming a second etching with the second mask layer to etch the firstoxide semiconductor layer, the second oxide semiconductor layer, and theconductive layer so that an oxide semiconductor layer having adepression, a source region, a drain region, a source electrode layer,and a drain electrode layer are formed, wherein the first mask layer isformed using a light-exposure mask, wherein the first etching is dryetching with use of an etching gas, wherein the second etching is wetetching with use of an etchant, and wherein the oxide semiconductorlayer having the depression includes a region with a smaller thicknessthan a region overlapping with the source region or the drain region. 8.The method for manufacturing a semiconductor device according to claim7, wherein each of the first oxide semiconductor film and the secondoxide semiconductor film includes indium, gallium, and zinc.
 9. Themethod for manufacturing a semiconductor device according to claim 7,wherein the first oxide semiconductor film has lower electricalconductivity than the second oxide semiconductor film.
 10. The methodfor manufacturing a semiconductor device according to claim 7, whereinthe etching gas includes chlorine.
 11. The method for manufacturing asemiconductor device according to claim 10, wherein the etching gasfurther includes oxygen.
 12. The method for manufacturing asemiconductor device according to claim 7, wherein an ammonia hydrogenperoxide mixture is used as the etchant in the second etching.
 13. Themethod for manufacturing a semiconductor device according to claim 7,wherein the conductive layer is etched using an ammonia hydrogenperoxide mixture as the etchant and the first oxide semiconductor layerand the second oxide semiconductor layer are etched using a mixedsolution of phosphoric acid, acetic acid, and nitric acid as the etchantin the second etching.
 14. The method for manufacturing a semiconductordevice according to claim 7, wherein a half-tone mask or a gray-tonemask is used as the light-exposure mask.